Process for preparing acrylic acid purified by crystallization from hydroxypropionic acid and apparatus therefore

ABSTRACT

The invention relates to a process for the preparation of acrylic acid and a process for the preparation of polyacrylic acid comprising the process steps: (a1) preparation of 3-hydroxypropionic acid from a biological material to give a fluid, in particular aqueous, phase containing 3-hydroxypropionic acid; (a2) dehydration of the 3-hydroxypropionic acid to give a fluid, in particular aqueous, solution containing acrylic acid; (a3) purification of the solution containing acrylic acid by a suspension crystallization or a layer crystallization to give a purified phase; and corresponding devices for carrying out these processes, and acrylic acid and polyacrylates. The invention is distinguished in that acrylic acid and polyacrylates can thereby be prepared efficiently, inexpensively, and sustainably with simple means and with a high purity on the basis of regenerable raw materials.

The present invention relates to a process for the preparation ofacrylic acid, a device for the preparation of acrylic acid, a processfor the preparation of polyacrylates, a device for the preparation ofpolyacrylates, the use of acrylic acid, and acrylic acid, polyacrylatesand chemical products containing these, in particular superabsorbers anddiapers.

Acrylic acid is a starting compound of great technical importance. Itserves inter alia for the preparation of polyacrylates, in particularcrosslinked, partly neutralized polyacrylates which have a largecapacity for absorption of water in the dry and substantially anhydrousstate. This can make up more than ten times its own weight. Because ofthe high absorption capacity, absorbent polymers are suitable forincorporation into water-absorbing structures and objects, such as e.g.baby diapers, incontinence products or sanitary napkins. These absorbentpolymers are also called “superabsorbers” in the literature. In thisconnection, reference is made to “Modern Superabsorbent PolymerTechnology”; F. L. Buchholz, A. T. Graham, Wiley-VCH, 1998.

Acrylic acid is conventionally obtained from propylene by a gas phaseoxidation which proceeds in two stages, in which propylene is firstoxidized to give acrolein, which is then reacted further to give acrylicacid. A disadvantage of this two-stage process for the preparation ofacrylic acid is on the one hand that the temperatures used in the tworeaction stages, which are conventionally between 300 and 450° C., leadto the formation of undesirable cracking products. This in turn resultsin an undesirably large amount of impurities being obtained, which mayalso be polymerizable and can be incorporated into the polymer backboneacid in the presence of crosslinking agents. This has an adverse effecton the properties of the superabsorbers. Aldehydes in particular, suchas, for example, furfural, acrolein or benzaldehyde, furthermore act asinhibitors in the free radical polymerization, with the consequence thatthe polymers still contain considerable amounts of soluble constituentsif they are not extracted from the acrylic acid employed for thepolymerization by elaborate purification steps.

It is also to be noted that if superabsorbers are employed in hygienearticles and in products for wound treatment, the toxic acceptabilityrequirement is very high. This eans that the educts employed for thepreparation of the superabsorbers likewise must have the highestpossible purities. It is therefore of great importance to provideacrylic acid as the main educt in an inexpensive manner in a form whichis as pure as possible for the preparation of superabsorbers.

A further disadvantage of the conventional process for the preparationof acrylic acid is that the educt employed (propylene) is prepared fromcrude oil and therefore from non-regenerating raw materials, which fromeconomic aspects above all is a disadvantage in the long term above allin view of the increasingly more difficult and above all more expensiveproduction of crude oil.

Here also, some approaches for counteracting this problem are alreadydescribed in the prior art. It is thus known in particular to obtainacrylic acid starting from hydroxypropionic acids, for example from2-hydroxypropionic acid or 3-hydroxypropionic acid, by dehydration ofthe hydroxypropionic acid.

The preparation of 2-hydroxypropionic acid by a fermentative route frombiomass, such as glucose or molasses, and by a synthetic route is knowninter alia from PEP Review 96-7 “Lactic acid by Fermentation” by RonaldBray of June 1998.

WO-A-03/62173 describes the preparation of 3-hydroxypropionic acid,which can serve inter alia as a starting substance for acrylic acidsynthesis. In this context, according to the teaching of WO-A-03/62173α-alanine is first formed fermentatively from pyruvate, and is thenconverted into beta-alanine by means of the enzyme 2,3-aminomutase. Theβ-alanine in turn is converted via β-alanyl-CoA, acrylyl-CoA,3-hydroxypropionyl-CoA or via malonic acid semialdehyde into3-hydroxypropionic acid, from which acrylic acid is obtained after adehydration.

WO-A-02/42418 describes a further route for the preparation of, forexample, 3-hydroxypropionic acid from regenerating raw materials. Inthis context, pyruvate is first converted into lactate, from whichlactyl-CoA is subsequently formed. The lactyl-CoA is then converted viaacrylyl-CoA and 3-hydroxypropionyl-CoA into 3-hydroxypropionic acid. Afurther route for the preparation of 3-hydroxypropionic acid describedin WO-A-02/42418 envisages the conversion of glucose via propionate,propionyl-CoA, acrylyl-CoA and 3-hydroxypropionyl-CoA. This publicationalso describes the conversion of pyruvate into 3-hydroxypropionic acidvia acetyl-CoA and malonyl-CoA. The 3-hydroxypropionic acid obtained bythe particular routes can be converted into acrylic acid by dehydration.

WO-A-01/16346 describes the fermentative preparation of3-hydroxypropionic acid from glycerol, in which microorganism whichexpress the dhaB gene from Klebsiella pneumoniae (a gene which codes forglycerol dehydratase) and a gene which codes for an aldehydedehydrogenase are employed. 3-Hydroxypropionic acid is formed fromglycerol via 3-hydroxypropionaldehyde in this manner, and can then beconverted into acrylic acid by dehydration.

Regardless of the enzymatic route by which the hydroxypropionic acid isobtained by fermentation, after the fermentative process an aqueouscomposition is present which, in addition to the hydroxypropionic acid,still contains numerous by-products, such as, for example, cells,non-converted biomass, salts and metabolism products formed alongsidethe hydroxypropionic acid.

In order to use the hydroxypropionic acid as a starting material for thepreparation of acrylic acid, it is advantageous first to concentrate andto purify this. In this context, in connection with the purification of3-hydroxypropionic acid it is known from WO-A-02/090312 first to addammonia to the fermentation solution for neutralization, in order toconvert the 3-hydroxypropionic acid into its ammonium salt. Thefermentation solution obtained in this way is subsequently brought intocontact with a high-boiling organic extraction agent and the mixture isheated, ammonia and water being stripped off in vacuo and the free3-hydroxypropionic acid formed being extracted into the organic phase.An organic phase containing 3-hydroxypropionic acid is obtained in thismanner, from which the 3-hydroxypropionic acid can be re-extracted or inwhich, after addition of a suitable catalyst, the 3 -hydroxypropionicacid can be converted into acrylic acid. The disadvantage of the “saltsplitting” process described in WO-A-02/090312 is, inter alia, that onthe one hand aqueous phase to be purified must contain the3-hydroxypropionic acid in an amount of at least 25 wt. % in order torender possible an effective purification by the so-called “saltsplitting process”. Since such high 3 -hydroxypropionic acidconcentrations cannot be achieved in the fermentation processescurrently known for the preparation of 3-hydroxypropionic acid, it isnecessary first to concentrate the 3-hydroxypropionic acid concentrationin the fermentation solution. This is conventionally effected byevaporating the water out of the fermentation solution. Furtherdisadvantages of the purification process described in WO-A-02/090312are that on the one hand this process envisages the addition of ahigh-boiling organic solvent to the fermentation solution, which must beseparated off from the 3 -hydroxypropionic acid in the further course ofthe process, and that on the other hand in the purification of3-hydroxypropionic acid from fermentation solutions which includenumerous organic by-products, these organic by-products are also atleast partly extracted into the organic solvent. In this case it isnecessary for the organic phase obtained in the salt splitting to bepurified further in order to obtain 3-hydroxypropionic acid with asatisfactory purity.

In connection with the purification of 2-hydroxypropionic acid (=lacticacid) from aqueous solutions, it is known from WO-A-95/024496 that theaqueous solution is first combined by means of an extraction agentcomprising a water-immiscible trialkylamine having a total of at least18 carbons in the presence of carbon dioxide under a partial pressure ofat least 345×10³ pascal to form an aqueous and an organic phase, and thelactic acid, which is in the organic phase, is subsequently extractedfrom the organic phase, for example by means of water. Here also thedisadvantage of the purification process is that in the case of afermentation solution as the starting composition, not only the lacticacid but also other by-products are dissolved in the organic phase, sothat no pure lactic acid solution is obtained in the re-extraction withwater.

In the dehydration of hydroxypropionic acid to give acrylic acid, wateris furthermore obtained. Even if quite concentrated hydroxypropionicacid solutions or even relatively pure hydroxypropionic acid areemployed, purification of an aqueous mixture of acrylic acid,hydroxypropionic acid and optional by-products obtained during thedehydration must therefore be carried out.

WO-A-2004/76398 proposes a vacuum distillation with dodecanol as anaddition for separation of a mixture of acrylic acid and3-hydroxypropionic acid. However, this gentle process is disadvantageousbecause of the use of dodecanol.

The present invention was based on the object of mitigating or evenovercoming in context the disadvantages emerging from the prior art.

In particular, the present invention was based on the object ofproviding a process for the preparation of acrylic acid from a fluidphase, preferably based on an aqueous fermentation solution, with whichan acrylic acid which is as pure as possible or an aqueous acrylic acidwhich is as pure as possible can be obtained under conditions which areas gentle as possible and simple. It should be possible for thispurification process to be carried out with the lowest possible or evenwithout the addition of organic solvents.

The present invention was furthermore based on the object of providing aprocess for the preparation of acrylic acid and a process for thepreparation of polyacrylate, which renders possible a preparation ofacrylic acid, in particular from biomass, which is as gentle and simpleas possible.

The present invention was also based on the object of providing devicesfor the preparation of acrylic acid and polymers thereof with which suchprocesses can be carried out.

SUMMARY

Overall, the aim of designing the particular process procedure asefficiently and inexpensively as possible to give end products which areas pure as possible is to be pursued, wherein as far as possibleregenerable raw materials are to be used as starting substances. Acontribution is thus to be made towards the preparation of acrylic acidbased on regenerating raw materials not only being more sustainable andecologically more advantageous than the conventional petrochemical routefor the preparation of acrylic acid, which usually proceeds viapropylene, but also being of economic interest.

A contribution towards achieving the abovementioned objects is madeaccording to the invention by the process for the preparation of acrylicacid, by the device for the preparation of acrylic acid, by the processfor the preparation of polyacrylates, by the device for the preparationof polyacrylates, and by the acrylic acid, polyacrylates and chemicalproducts containing these, in particular superabsorbers and diapers, asdescribed in the particular main and secondary claims. Furtherembodiments and developments, each of which can be combined individuallyor as desired with one another, are the subject matter of the particulardependent claims.

The process according to the invention for the preparation of acrylicacid comprises the process steps:

-   (a1) provision of hydroxypropionic acid, preferably of    2-hydroxypropionic acid or 3-hydroxypropionic acid, most preferably    of 3-hydroxypropionic acid, from a biological material, to give a    fluid F1 containing hydroxypropionic acid, preferably    2-hydroxypropionic acid or 3-hydroxypropionic acid, most preferably    3-hydroxypropionic acid, in particular an aqueous phase P1,-   (a2) dehydration of the hydroxypropionic acid, preferably the    2-hydroxypropionic acid or 3-hydroxypropionic acid, most preferably    the 3-hydroxypropionic acid, to give a fluid F2 containing acrylic    acid, in particular an aqueous phase P2,-   (a3) purification of the fluid F2 containing acrylic acid,    preferably the aqueous phase P2, by a suspension crystallization or    a layer crystallization, to give a purified phase.

The terms “acrylic acid”, “hydroxypropionic acid”, “2-hydroxypropionicacid” and “3-hydroxypropionic acid” as used herein always describe inthis context the corresponding carboxylic acid in that form in whichthey are present in the fluids F1 or

F2 under the given pH conditions. The terms therefore always include thepure acid form (acrylic acid, hydroxypropionic acid, 2-hydroxypropionicacid or 3-hydroxypropionic acid), the pure base form (acrylate,hydroxypropionate, 2-hydroxypropionate or 3-hydroxypropionate) andmixtures of the protonated and deprotonated form of the acids.

In the process according to the invention it is preferable for at leastone, preferably at least two of the steps of the process according tothe invention to be carried out continuously and not to have to beconstantly interrupted by batchwise reactions and started up again.Preferably, at least the steps of dehydration and of crystallization andparticularly preferably all the steps are carried out continuously.

According to a particular embodiment of the preparation processaccording to the invention, the fluid F1 initially introduced in processstep (a1) is an aqueous phase P1, this aqueous phase P1 preferably beingobtained by a process comprising the process steps:

-   i) preparation, preferably fermentative preparation, of    hydroxypropionic acid from a biological material, particularly    preferably from carbohydrates, in particular from glucose, or from    glycerol, in an aqueous composition to give an aqueous phase    containing hydroxypropionic acid and microorganisms,-   ii) optionally killing of the microorganisms, preferably by heating    this aqueous phase to temperatures of at least 100° C., particularly    preferably at least 110° C. and moreover preferably at least 120° C.    for a duration of at least 60 seconds, for example 10 minutes and/or    at least 30 minutes,-   iii) optionally separating off of solids, in particular of    microorganisms or unreacted biological material, from the aqueous    phase, preferably by means of sedimentation, centrifugation or    filtration.

Preferably, recombinant microorganisms, particularly preferablyrecombinant bacterial, fungal or yeast cells, are employed for thepreferably fermentative preparation of the hydroxypropionic acid from abiological material in process step i). Recombinant microorganisms whichare particularly preferred according to the invention are bacteria ofthe genera Corynebacterium, Brevibacterium, Bacillus, Lactobacillus,Lactococcus, Candida, Pichia, Kluveromyces, Saccharomyces, Bacillus,Escherichia and Clostridium, Bacillus flavum, Bacillus lactofermentum,Escherichia coli, Saccharomyces cerevisiae, Kluveromyces lactis, Candidablankii, Candida rugosa, Corynebacterium glutamicum, Corynebacteriumefficiens and Pichia postoris being moreover preferred andCorynebacterium glutamicum being most preferred.

In this context, it is furthermore preferable according to the inventionfor the microorganisms to be genetically modified such that comparedwith their wild-type they show a formation of hydroxypropionic acid froma biological material, preferably from carbohydrates, such as glucose,or from glycerol, which is increased, preferably a formation which isincreased by a factor of at least 2, particularly preferably of at least10, moreover preferably of at least 100, moreover still more preferablyof at least 1,000 and most preferably of at least 10,000. In thiscontext, this increase in the hydroxypropionic acid formation can inprinciple be effected via all the metabolic pathways known to the personskilled in the art, in the case of the formation of 3-hydroxypropionicacid in particular via the routes described in the publicationsWO-A-03/62173, WO-A-02/42418 and WO-A-01/16346 and in the case of theformation of 2-hydroxypropionic acid in particular from bacteria strainsof the genus Bacillus or Lactobacillus, for example in the mannerdescribed in DE 40 00 942 C2. The formation of hydroxypropionic acids,in particular of 2- or 3-hydroxypropionic acids, is particularlypreferably carried out by means of recombinant microorganisms in whichthe activity of one or more enzymes relevant for the formation of thecorresponding hydroxypropionic acids has been increased, it beingpossible for the increase in the enzyme activities to be effected bymeasures known to the person skilled in the art, in particular bymutation or increasing the gene expression.

The genetically modified microorganisms can be brought into contact witha suitable nutrient medium, and therefore cultured, continuously ordiscontinuously in the batch process (batch culturing) or in the fedbatch process (feed process) or repeated fed batch process (repetitivefeed process) for the purpose of the production of hydroxypropionicacid. A semi-continuous process such as is described in GB-A-1009370 isalso conceivable. A summary of known culturing methods is described inthe textbook by Chmiel (“Bioprozesstechnik 1. Einführung in dieBioverfahrenstechnik [Bioprocess technology 1. Introduction tobioprocess technology]” (Gustav Fischer Verlag, Stuttgart, 1991)) or inthe textbook by Storhas (“Bioreaktoren and periphere Einrichtungen[Bioreactors and peripheral equipment]”, Vieweg Verlag,Braunschweig/Wiesbaden, 1994).

The culture medium to be used must meet the requirements of theparticular strains in a suitable manner. Descriptions of culture mediaof various microorganisms are contained in the handbook “Manual ofMethods for General Bacteriology” of the American Society forBacteriology (Washington D.C., USA, 1981).

Sugars and carbohydrates, such as e.g. glucose, sucrose, lactose,fructose, maltose, molasses, starch and cellulose, oils and fats, suchas e.g. soya oil, sunflower oil, groundnut oil and coconut fat, fattyacids, such as e.g. palmitic acid, stearic acid and linoleic acid,alcohols, such as e.g. glycerol and ethanol, and organic acids, such ase.g. acetic acid, can be used as biological material, in particular as asource of carbon. These substances can be used individually or as amixture. The use of carbohydrates, in particular monosaccharides,oligosaccharides or polysaccharides, as is described in U.S. Pat. No.6,01,494 and U.S. Pat. No. 6,136,576, of C5 sugars or of glycerol isparticularly preferred.

Organic nitrogen-containing compounds, such as peptones, yeast extract,meat extract, malt extract, corn steep liquor, soybean flour and urea,or inorganic compounds, such as ammonium sulphate, ammonium chloride,ammonium phosphate, ammonium carbonate and ammonium nitrate, can be usedas a source of nitrogen. The sources of nitrogen can be usedindividually or as a mixture.

Phosphoric acid, potassium dihydrogen phosphate or dipotassium hydrogenphosphate or the corresponding sodium-containing salts can be used as asource of phosphorus. The culture medium must furthermore contain saltsof metals, such as e.g. magnesium sulphate or iron sulphate, which arenecessary for growth. Finally, essential growth substances, such asamino acids and vitamins, can be employed in addition to theabovementioned substances. Suitable precursors can moreover be added tothe culture medium. The starting substances mentioned can be added tothe culture in the form of a one-off batch or can be fed in during theculturing in a suitable manner.

Basic compounds, such as sodium hydroxide, sodium bicarbonate, potassiumhydroxide, ammonia or aqueous ammonia, or acidic compounds, such asphosphoric acid or sulphuric acid, are employed in a suitable manner tocontrol the pH of the culture, the addition of ammonia beingparticularly preferred. Antifoam agents, such as e.g. fatty acidpolyglycol esters, can be employed to control the development of foam.Suitable substances having a selective action, such as e.g. antibiotics,can be added to the medium to maintain the stability of plasmids. Oxygenor oxygen-containing gas mixtures, such as e.g. air, are introduced intothe culture in order to maintain aerobic conditions. The temperature ofthe culture is usually 20° C. to 45° C., and preferably 25° C. to 40° C.

In connection with this particular embodiment of the process accordingto the invention, it is preferable for the aqueous phase P1 prepared inprocess step (a1) to have a composition Cl comprising

-   (C1_(—)1) 1 to 40 wt. %, preferably 5 to 30 wt. % and most    preferably 10 to 20 wt. % of hydroxypropionic acid, preferably 2- or    3-hydroxypropionic acid and particularly preferably    3-hydroxypropionic acid, salts of these acids or mixtures thereof,-   (C1_(—)2) 0.1 to 5 wt. %, preferably 0.3 to 2.5 wt. % and most    preferably 0.5 to 1 wt. % of inorganic salts,-   (C1_(—)3) 0.1 to 30 wt. %, preferably 0.5 to 20 wt. % and most    preferably 1 to 10 wt. % of organic compounds which differ from the    hydroxypropionic acid,-   (C1_(—)4) 0 to 50 wt. %, preferably 1 to 40 wt. %, preferably 5 to    20 wt. % and most preferably 1 to 10 wt. % of solids, in particular    solids of fine plant parts or cells and/or cell fragments, in    particular of microorganisms or unreacted biological material, and-   (C1_(—)5) 20 to 90 wt. %, preferably 30 to 80 wt. % and most    preferably 40 to 70 wt. % of water,    wherein the sum of components (C1_(—)1) to (C1_(—)5) is 100 wt. %.

This fluid phase preferably has a pH in a range of from 5 to 8,preferably 5.2 to 7 and particularly preferably 5.5 to 6.5.

The salt of the hydroxypropionic acid is preferably the sodium salt, thepotassium salt, the calcium salt, the ammonium salt or mixtures thereof,the ammonium salt being particularly preferred.

The inorganic salts are preferably chosen from the group containingsodium chloride, potassium chloride, phosphates, such as sodiumphosphate, sodium carbonate, sodium bicarbonate, sodium dihydrogenphosphate or disodium hydrogen phosphate, magnesium sulphate, ironsulphate, calcium chloride, calcium sulphate or ammonium salts, such asammonium sulphate, ammonium chloride, ammonium phosphate, ammoniumcarbonate or ammonium nitrate.

The organic compounds which differ from the hydroxypropionic acidcomprise, inter alia, unreacted biological material, such as, forexample, carbohydrates or glycerol, metabolism products, such as, forexample, lactate, pyruvate or ethanol, antibiotics, antifoam agents,organic buffer substances, such as, for example, HEPES, amino acids,vitamins, peptones, urea or nitrogen-containing compounds, such as arecontained, for example, in yeast extract, meat extract, malt extract,corn steep liquor and soybean flour.

The aqueous fluid F1 obtained after the fermentation and containinghydroxypropionic acid and microorganisms or the aqueous phase P1(=fermentation solution) can be heat treated in a further process stepii) in order to kill the microorganisms still contained therein. This ispreferably effected by heating this aqueous phase P1 to temperatures ofat least 100° C., particularly preferably at least 110° C. and moreoverpreferably at least 120° C. for a duration of at least 60 seconds,preferably at least 10 minutes and moreover preferably at least 30minutes, the heating preferably being carried out in devices known forthis to the person skilled in the art, such as, for example, anautoclave. Killing of the microorganisms by high-energy radiation, suchas, for example, UV radiation, is also conceivable, killing of themicroorganisms by heating being particularly preferred.

In this particular embodiment of the process according to the invention,it may furthermore be preferable for solids, in particular fine plantparts or cells and/or cell fragments, in particular microorganisms orunreacted biological material, to be separated off from the aqueousphase P1 obtained after the fermentation in a further process step iii)before, during or after the killing of the microorganisms in processstep ii), it being possible for this separating off to be carried out byall the processes known to the person skilled in the art for separatingoff solids from liquid compositions, but preferably by sedimentation,centrifugation or filtration, separating off by filtration being mostpreferred. In this context, all the filtration processes which arementioned in chapter 4.1.3 “Filtrieren and Auspressen [Filtration andpressing off]” in “Grundoperationen Chemischer Verfahrenstechnik [Basicoperations of chemical process technology]”, Wilhelm R. A. Vauck andHermann A. Muller, WILEY-VCH-Verlag, 11th revised and extended edition,2000, and seem suitable to the person skilled in the art for separatingoff solids, in particular microorganisms, from fermentation solutionscan be used.

Before the fermentation solution obtained in this manner and optionallyfreed from solids is subjected to process step (a2) as the aqueous phaseP1, it may also be appropriate to reduce the water content of thisaqueous phase, in particular by a factor of less than 0.8, in particularless than 0.5, for example less than 0.3, by means of evaporation ordistillation, reverse osmosis, electroosmosis, azeotropic distillation,electrodialysis or multiple phase separation. In this context, areduction by a factor of less than 0.8 means that after the reductionthe water content of the fermentation solution is less than 80% of thewater content of the fermentation solution before the reduction.

In particular, it is preferable according to the invention for the watercontent to be reduced to the extent that the concentration ofhydroxypropionic acid or of salts thereof in the fermentation solutionis at least 10 wt. %, particularly preferably at least 20 wt. %,moreover preferably at least 30 wt. % and most preferably at least 40wt. %.

In a modification of the invention, the process for the preparation ofthe fluid F1 or of the aqueous phase P1 furthermore comprises theprocess step (iv), in which at least a part of the water in thefermentation solution is separated off, this separating off preferablybeing carried out by the formation of a first phase and a second phasefrom the fluid F1, in particular from the aqueous phase P1. However, inthe context of the process according to the invention it may benecessary for yet a further purification to be advantageous as processstep iv), instead of or in addition to a separating off of solids iii).All the measures which are known to the person skilled in the art forthis purpose and seem suitable are in principle possible aspurifications. This is preferably chosen from a group consisting of saltprecipitation, esterification, membrane and sorption processes or acombination of two. Of the membrane processes, micro-, ultra- ornanofiltration, osmosis, in particular reverse osmosis, orelectrodialysis, or a combination of at least two of these arepreferred. Preferred sorption processes are ion exchange, chromatographyor extraction, preferably reactive extraction, or a combination of atleast two of these. In the case of extraction, the use of hypercriticalfluids, for example of CO₂, often in the presence of amines, is known.It is furthermore preferable for this formation of the first and secondphase to be carried out by at least one or also at least two or more ofthe following separation processes:

-   -   evaporation or distillation,    -   reverse osmosis,    -   electroosmosis,    -   azeotropic distillation,    -   electrodialysis,    -   multiple phase separation,        wherein the first and the second phase have different        concentrations of acrylic acid, and after the formation of these        two phases the phases are separated from one another to give a        purified phase containing acrylic acid.

Process step (iv) can in principle be carried out before process step(a2) or, such as, for example, in the reactive distillation describedbelow, at the same time as process step (a2). By process step (iv), aprepurified first or second phase is formed from the fluid F1, which isthen subjected to the dehydration reaction in process step (a2).

All the devices which are mentioned in chapter 10.1 “Verdampfen[Evaporation]” in “Grundoperationen Chemischer Verfahrenstechnik [Basicoperations of chemical process technology]”, Wilhelm R. A. Vauck andHermann A. Muller, WILEY-VCH-Verlag, 11th revised and extended edition,2000, and seem suitable to the person skilled in the art for evaporationof water from a fermentation solution freed from solids can be employedas the evaporation device.

Reverse osmosis—also called hyperfiltration—is a pressure filtration ona semipermeable membrane in which, in particular, a pressure is used inorder to reverse the natural osmosis process. In this context, thepressure to be applied must be greater than the pressure which wouldarise due to the osmotic requirement for balancing the concentration.The osmotic membrane, which allows through only the carrier liquid(solvent) and retains the dissolved substances (solutes), must bedesigned to withstand this often high pressure. When the pressuredifference more than compensates the osmotic gradient, the solventmolecules pass through the membrane, as in a filter, while the impuritymolecules are retained. The osmotic pressure increases with increasingconcentration difference, and stops when the natural osmotic pressure isequal to the preset pressure. In a continuous procedure, the concentrateis removed continuously. Flow-through membranes can be used. In thiscontext, the fluid F1 is pressed against a pore membrane, preferably ofcellulose acetate or polyamide, under pressures of as a rule 2 to 15MPa, water, but not the hydroxypropionic acid or salt thereof, beingable to pass through the pores of the membrane. Suitable membranematerials and devices can be found, inter alia, in chapter 10.7“Permeation” in “Grundoperationen Chemischer Verfahrenstechnik [Basicoperations of chemical process technology]”, Wilhelm R. A. Vauck andHermann A. Muller, WILEY-VCH-Verlag, 11th revised and extended edition,2000.

In electroosmosis, the electroosmotic pressure, with which a separationof the hydroxypropionic acid from an impurity, such as e.g. water, canlikewise take place, is established in an equilibrium state. Inelectroosmosis, water is likewise separated off selectively from anaqueous composition via a semipermeable membrane, but in contrast toreverse osmosis the water is driven through the semipermeable membranenot via an increased pressure but by application of an electricalvoltage of as a rule 6 to 20 V.

In azeotropic distillation or azeotropic rectification, an azeotropicmixture is separated by addition of a third component. Rectification isa thermal separation process and represents a further development ofdistillation or a succession of many distillation steps. The essentialadvantages of rectification are that the installation can be operatedcontinuously and that the separating effect is many times highercompared with distillation, since the vapour is in contact with theliquid. As a result, the higher-boiling content condenses and the morereadily volatile constituents of the liquid phase evaporate due to theheat of condensation liberated. The contact area between the vapourphase and liquid phase is provided by built-in components (e.g. bubbletrays). As is also the case in distillation, under normal conditionsonly non-azeotropic mixtures can be separated. If it is necessary toseparate an azeotropic mixture, the azeotropic point is shifted andshould not lie in the concentration and temperature range of theinstallation. A shift is effected e.g. by changing the operatingpressure or by addition of a particular auxiliary substance. Azeotropicdistillation is carried out in particular using organic solvents, suchas e.g. toluene or dodecanol, as an entraining agent which forms anazeotrope with water. The aqueous solution brought into contact with theorganic solvent is subsequently distilled, water and the organic solventbeing separated off from the composition.

Electrodialysis is understood as meaning splitting of a chemicalcompound under the action of an electrical current. In electrodialysis,two electrodes are immersed in the aqueous phase, formation of hydrogenoccurring at the cathode and formation of oxygen occurring at the anode.Two phases are likewise formed in this manner, namely the aqueous phasefreed from at least a part of the water, and the gas phase comprisinghydrogen and oxygen. In this case in particular one of the components(e.g. water) containing the fluid phase is converted into itsconstituents, which can be removed as a gas or (e.g. by means of aprecipitation reaction effected after the electrolytic reaction or aconcentration of solid at an electrode) as a solid.

In multiple phase separation, such as is described, for example, inWO-A-02/090312, in a first process step the fluid F1, in particular theaqueous phase P1, in which the hydroxypropionic acid has preferably beenat least partly converted into the ammonium salt by the addition ofammonia, is brought into contact with a high-boiling, preferablywater-immiscible organic solvent or with a high-boiling solvent mixture.The composition which has been brought into contact with thehigh-boiling, preferably water-immiscible organic solvent or with ahigh-boiling solvent mixture is subsequently heated to a temperature offrom preferably 20 to 200° C., particularly preferably 40 to 120° C.,ammonium and water vapour escaping from the composition, and thehydroxypropionic acid is extracted in the acid form into thehigh-boiling organic solvent or into the high-boiling solvent mixtureand an organic phase containing hydroxypropionic acid is obtained. Thisprocess, which is also called “salt splitting”, is described inWO-A-02/090312. The organic phase obtained in this manner can beemployed as fluid F1 in process step (a2).

In this context, those solvents which are mentioned as preferredextraction agents in WO-A-02/090312 are employed as preferred organicsolvents. Preferred high-boiling organic solvents in this context areorganic amines, in particular trialkylamines having a total number ofcarbon atoms of at least 18 and a boiling point of at least 100° C.,preferably at least 175° C., under atmospheric pressure, such astrioctylamine, tridecylamine or tridodecylamine, and solvent mixture ofthese trialkylamines and high-boiling carbon-oxygen compounds, such as,for example, alcohols, high-boiling phosphorus-oxygen compounds, such asphosphoric acid esters, high-boiling phosphine sulphides or high-boilingalkyl sulphides, “high-boiling” preferably meaning that these compoundshave a boiling point of at least 175° C. under atmospheric pressure.

After the fluid F1, in particular the aqueous phase P1, containing theammonium salt of the hydroxypropionic acid has been brought into contactwith these solvents or solvent mixture in accordance with the proceduredescribed in WO-A-02/090312, the composition obtained in this way isheated, water vapour and ammonia escaping. The release of thesecomponents, which are at least partly in gaseous form under thetemperature conditions, can optionally be promoted by a reducedpressure. Finally, an organic phase which contains the hydroxypropionicacid in its acid form and which can then be employed as fluid F1 inprocess step (a2) is obtained. It is also conceivable to re-extract thehydroxypropionic acid with water and to subject the aqueoushydroxypropionic acid solution obtained in this way to process step(a2).

A multiple phase separation such as is described, for example, inWO-A-95/024496 is furthermore conceivable. According to this separationprocess, a hydroxycarboxylic acid solution in which thehydroxycarboxylic acid is present as hydroxypropionate due to theaddition of basic compounds, such as, for example, sodium bicarbonate,is combined, optionally after a filtration and after the water has beenevaporated off, by means of an extraction agent comprising awater-immiscible trialkylamine having a total of at least 18 carbons inthe presence of carbon dioxide under a partial pressure of at least345×10³ pascal to form an aqueous and an organic phase, and thehydroxypropionic acid, which is in the organic phase, is subsequentlyextracted from the organic phase, for example by means of water. In thismultiple phase separation process also, both the organic phasecontaining hydroxypropionic acid and the aqueous phase which containshydroxypropionic acid and is obtained after a re-extraction with watercan be subjected to process step (a2).

The dehydration of the hydroxypropionic acid and therefore the synthesisof acrylic acid in process step (a2) is preferably carried out byheating the aqueous fermentation solution, which optionally has beenfreed from microorganisms and has optionally been dewatered, or theorganic or aqueous phase obtained by applying multiple phase separation,this heating particularly preferably being carried out in the presenceof a catalyst.

Both acidic and alkaline catalysts are possible dehydration catalysts.Acidic catalysts are preferred in particular because of the low tendencytowards oligomer formation. The dehydration catalyst can be employedboth as a homogeneous and as a heterogeneous catalyst. If thedehydration catalyst is present as a heterogeneous catalyst, it ispreferable for the dehydration catalyst to be in contact with a supportx. All the solids which seem suitable to the person skilled in the artare possible as the support x. In this connection, it is preferable forthese solids to have suitable pore volumes which are suitable for goodbinding and uptake of the dehydration catalyst. Total pore volumesaccording to DIN 66133 in a range of from 0.01 to 3 ml/g are furthermorepreferred, and those in a range of from 0.1 to 1.5 m¹/g are particularlypreferred. It is additionally preferable for the solids suitable as thesupport x to have a surface area in the range of from 0.001 to 1,000m²/g, preferably in the range of from 0.005 to 450 m²/g and moreoverpreferably in the range of from 0.01 to 300 m²/g according to the BETtest in accordance with DIN 66131. On the one hand bulk material whichhas an average particle diameter in the range of from 0.1 to 40 mm,preferably in the range of from 1 to 10 mm and moreover preferably inthe range of from 1.5 to 5 mm can be employed as the support for thedehydration catalyst. The wall of the dehydration reactor canfurthermore serve as the support. Moreover, the support can be acidic orbasic per se, or an acidic or basic dehydration catalyst can be appliedto an inert support. Application techniques which may be mentioned are,in particular, immersion or impregnation, or incorporation into asupport matrix.

Suitable supports x, which can also have dehydration catalystproperties, are, in particular, natural or synthetic silicaticsubstances, such as, in particular, mordenite, montmorillonite, acidiczeolites, acidic aluminium oxides, γ-Al₂O₃, support substances, such asoxidic or silicatic substances, for example Al₂O₃, TiO₂, coated withmono-, di- or polybasic inorganic acids, in particular phosphoric acid,or acidic salts of inorganic acids; oxides and mixed oxides. such as,for example, gamma-Al₂O₃ and ZnO—Al₂O₃ mixed oxides of theheteropolyacids.

In one embodiment according to the invention, the support x comprises atleast in part an oxidic compound. Such oxidic compounds should containat least one of the elements from Si, Ti, Zr, Al and P or a combinationof at least two of these. Such supports can also themselves act as thedehydration catalyst due to their acidic or basic properties. Apreferred class of compounds acting both as a support as x and as adehydration catalyst contains silicon-aluminium-phosphorus oxides.Preferred basic substances which function both as a dehydration catalystand as a support x contain alkali metal, alkaline earth metal,lanthanum, lanthanoids or a combination of at least two of these intheir oxidic form, such as, for example, Li₂O—, Na₂O—, K₂O—, Cs₂O—,MgO—, CaO—, SrO— or BaO— or La₂O₃-containing substances. Such acidic orbasic dehydration catalysts are commercially obtainable both fromDegussa AG and from Süchemie AG. Ion exchangers represent a furtherclass. These can also be either in basic or in acidic form.

Possible homogeneous dehydration catalysts are, in particular, inorganicacids, preferably phosphorus-containing acids, and moreover preferablyphosphoric acid. These inorganic acids can be immobilized on the supportx by immersion or impregnation.

The use of heterogeneous catalysts has proved particularly suitable ingas phase dehydration in particular. In liquid phase dehydration,however, both homogeneous and heterogeneous dehydration catalysts areemployed.

It is furthermore preferable for a dehydration catalyst having an H₀value in the range of from +1 to −10, preferably in a range of from +2to −8.2 and moreover preferably in the case of the liquid phasedehydration in a range of from +2 to −3 and in the gas phase dehydrationin a range of from −3 to −8.2 bar to be employed in the processaccording to the invention for the preparation of acrylic acid. The H₀value corresponds to the Hammett acidity function and can be determinedby so-called amine titration and use of indicators or by absorption of agaseous base—see “Studies in Surface Science and Catalytics”, vol. 51,1989: “New solid acids and bases, their catalytic properties”, K.Tannabe et al. Further details on the preparation of acrolein fromglycerol are furthermore to be found in DE 42 38 493 C1.

According to a particular embodiment of the process according to theinvention for the preparation of acrylic acid, a porous support bodywhich is preferably based to the extent of at least 90 wt. %, moreoverpreferably to the extent of at least 95 wt. % and most preferably to theextent of at least 99 wt. % on a silicon oxide, preferably on SiO₂, andhas been brought into contact with an inorganic acid, preferably withphosphoric acid, or with super-acids, such as, for example, sulphated orphosphated zirconium oxide, is employed as an acidic solid catalyst. Theporous support body is preferably brought into contact with theinorganic acid by impregnation of the support body with the acid, thispreferably being brought into contact with the support body in an amountin a range of from 10 to 70 wt. %, particularly preferably in a range of20 to 60 wt. % and moreover preferably in a range of from 30 to 50 wt.%, based on the weight of the support body, and then being dried. Afterthe drying, the support body is heated for fixing of the inorganic acid,preferably to a temperature in a range of from 300 to 600 ° C., moreoverpreferably in a range of from 400 to 500° C.

According to a particular embodiment of the process according to theinvention, the dehydration of the hydroxypropionic acid in process step(a2) is carried out by a liquid phase dehydration, most preferably bymeans of a so-called “reactive distillation”.

“Reactive distillation” is a reaction from which one component isseparated off by evaporation and the chemical equilibrium of thisreaction is thereby influenced. In particular, simultaneous carrying outof a chemical reaction and a distillation in one column, in particular acountercurrent column, is called “reactive distillation”. Thissimultaneous carrying out of the reaction and substance separation isparticularly advantageous for those reactions in which the educts arenot converted completely into the desired products due to the positionof the chemical equilibrium. By the simultaneous separating off of thereaction products from the reaction space, an almost complete reactionalso takes place in these cases in a single apparatus. Furtheradvantages of reactive distillation are suppression of undesirable sidereactions, thermal utilization of an exothermic heat of reaction for thedistillation and facilitation of the subsequent working up of theproduct. In connection with the dehydration of hydroxypropionic acid,the term “reactive distillation” accordingly preferably describes aprocess in which the fluid F1 containing the hydroxypropionic acid, orthe aqueous phase P1, which has been obtained in process step (a1),optionally after a further treatment by means of one or all of theprocess steps (i) to (iv), is heated in the presence of adehydration-promoting catalyst under conditions under which thehydroxypropionic acid is at least partly converted into acrylic acid andunder which at the same time water in the reaction solution can bedistilled off. In this manner, the hydroxypropionic acid is dehydratedto give acrylic acid and the reaction mixture is distilled at the sametime, water being separated off as the distillate and a bottom productcontaining acrylic acid being obtained.

In order to avoid decarboxylation reactions during the reactivedistillation, it is furthermore particularly advantageous to carry outthis reactive distillation in a CO₂ atmosphere. This furthermore has theadvantage that the CO₂ is present in the aqueous reaction mixture atleast partly in the form of carbonic acid, and as an acid at the sametime promotes the dehydration reaction as a catalyst. In this context,the term “CO₂ atmosphere” is preferably understood as meaning anatmosphere which contains at least 10 vol. %, particularly preferably atleast 25 vol. % and most preferably at least 50 vol. % of CO₂.

All the distillation or rectification devices known to the personskilled in the art can be employed as reactors for the reactivedistillation. In this context, in the case of a heterogeneous catalysisthe catalyst can be immobilized on a suitable support inside thesedistillation or rectification devices, for example by using structuredpackings, such as are obtainable, for example, under the name“Katapak-SP” from Sulzer-Chemtech, or by using thermal sheets coatedwith catalyst, and in the case of a homogeneous catalysis it can also beintroduced into the inside of the distillation or rectification devicesvia a suitable inlet. Packed columns which contain packing bodies, suchas, for example, hollow cylindrical packing materials, for exampleRASCHIG rings, INTOS rings, PALL rings, wire mesh rings, extended jacketrings, coiled rings, WILSON spiral rings or PYM rings, reel-shapedpacking bodies, such as, for example, HALTMEIER rolls, saddle-shapedpacking bodies, such as, for example, BERL saddles, INTALOX saddles orwire mesh saddles, cross-shaped packing bodies, such as, for example,twinned bodies, propeller bodies or star-shaped bodies, box-like packingbodies, such as, for example, HELI-PAK bodies or OCTA-PAK bodies, orspherical packing bodies, such as, for example, ENVI-PAK bodies, orplate columns can furthermore be employed in principle, it beingpossible for the packing bodies described above to be at least partlycoated with catalyst. Packed columns have a very low liquid contentcompared with plate columns. This is indeed often advantageous for therectification, since the risk of thermal decomposition of the substancesis thereby reduced. However, the low liquid content of packed columns isa disadvantage for reactive distillation, especially in the case ofreactions having a finite rate of reaction. In order to achieve thedesired conversions in slow reactions, a long dwell time of the liquidin the apparatus must therefore be ensured. The use of plate columns cantherefore be particularly advantageous according to the invention. Thesehave a high liquid content, and indeed both in the two-phase layer onthe plate and in the downcomers. Both portions of the liquid content canbe modified in a targeted manner and adapted to the particularrequirements of the reactive distillation by construction measures, i.e.by the design of the plates. The catalyst can furthermore be distributedover the entire length, that is to say from the bottom region to the topregion, of the distillation or rectification devices. However, it isalso conceivable for the catalyst to be located only in a particularregion, preferably in the lower half, particularly preferably in thelower third and most preferably in the lower quarter of the distillationor rectification device.

In the case of the abovementioned reactive distillation, the processaccording to the invention preferably includes the following processsteps:

-   (a1) preparation of hydroxypropionic acid, preferably of    2-hydroxypropionic acid or 3-hydroxypropionic acid, most preferably    3-hydroxypropionic acid, from a biological material, to give an    aqueous phase P1 containing hydroxypropionic acid, preferably    2-hydroxypropionic acid or 3-hydroxypropionic acid, most preferably    3-hydroxypropionic acid, wherein the preparation of the aqueous    phase P1 includes the following process steps:    -   i) preparation, preferably fermentative preparation, of        hydroxypropionic acid from a biological material, particularly        preferably from carbohydrates, in particular from glucose, or        from glycerol, in an aqueous composition to give an aqueous        phase containing hydroxypropionic acid and microorganisms,        -   ii) optionally killing of the microorganisms, preferably by            heating this aqueous phase to temperatures of at least 100°            C., particularly preferably at least 110° C. and moreover            preferably at least 120° C. for a duration of at least            60seconds, for example 10 minutes and/or at least 30            minutes, and        -   iii) optionally separating off of solids, in particular of            microorganisms or unreacted biological material, from the            aqueous phase, preferably by means of sedimentation,            centrifugation or filtration,        -   iv) optionally at least partly separating off the water from            the aqueous phase,-   (a2) dehydration of the hydroxypropionic acid, preferably the    2-hydroxypropionic acid or 3-hydroxypropionic acid, most preferably    the 3-hydroxypropionic acid, by means of a reactive distillation,    preferably under a CO₂ atmosphere, to give a fluid F2 containing    acrylic acid, in particular an aqueous phase P2, as the bottom    product,-   (a3) purification of the bottom product containing acrylic acid by a    suspension crystallization or a layer crystallization, to give a    purified phase.

In order to ensure the longest possible dwell time of the fluid F1 or ofthe aqueous phase P1 in the dehydration reactors, it may be advantageousto employ, in addition to the distillation or rectification device, atleast one further reactor in which at least a part of thehydroxypropionic acid contained in the fluid F1 or in the aqueous phaseP1 is converted into acrylic acid, before the fluid F1 or the aqueousphase P1 is introduced into the distillation or rectification device forfurther reaction.

In connection with the process according to the invention describedabove, especially if a reactive distillation is carried out, it istherefore particularly preferable for the dehydration to be carried outat least in two reactors, the second and optionally each further reactorbeing designed as a reactive distilling device. In this context, theaqueous phase P1—preferably freed from solids—is heated in at least onefirst reactor R1, which is preferably not constructed as a distillationor rectification device, in the presence of a homogeneous orheterogeneous catalyst, preferably an inorganic acid, particularlypreferably phosphoric acid, to give a first aqueous fluid F1_1containing acrylic acid, preferably a first aqueous phase P1_1containing acrylic acid, this heating being carried out under a pressureΠ1. This fluid F1_1 or this aqueous phase P1_1 is subsequentlyintroduced into a further reactor R2, which likewise contains ahomogeneous or heterogeneous catalyst and is preferably constructed as adistillation or rectification column. The further dehydration ofhydroxypropionic acid still present is then carried out in this reactorR2 by heating the fluid F1_1 in the presence of a homogeneous orheterogeneous catalyst to give a fluid F1_2 containing acrylic acid,preferably an aqueous phase P1_2 containing acrylic acid, as the bottomproduct, this heating being carried out under a pressure Π2, wherein thepressure Π2 is generally different from the pressure Π1 and in apreferred embodiment is lower than Π1, preferably by at least 0.1 bar,particularly preferably at least 1 bar, moreover preferably at least 2bar, in addition preferably at least 4 bar, furthermore preferably atleast 8 bar and moreover preferably at least 10 bar lower than Π1. Inone embodiment of the process according to the invention, the pressureof the first reactor R1 is in a range of from 4.5 to 25 bar, preferablyin a range of from 5 to 20 bar and moreover preferably in a range offrom 6 to 10 bar and the pressure of the further reactor R2 is in arange of from 1 to <4.5 bar and preferably in a range of from 2 to 4bar. In one embodiment of the process according to the invention, thepressure of the first reactor R1 is in a range of from 4.5 to 25 bar,preferably in a range of from 5 to 20 bar and moreover preferably in arange of from 6 to 10 bar and the pressure of the further reactor R2 isin a range of from 0.01 to <1 bar and preferably in a range of from 0.1to 0.8 bar. Water is separated off as the distillate in this reactionvessel R2. In the reactive distillation, water often leaves overhead inan amount of 20 wt. % and more, preferably 40 wt. % and more andparticularly preferably 60 wt. % and more. The remaining water remainsin the bottom product of the reactive distillation. The above wt. % arein each case based on 100 wt. % of the component introduced into thereactive column.

In this connection, it is furthermore preferable for both the reactor R1and the reactor R2 to be gassed with CO₂ before the introduction of thefluid F1, of the aqueous phase P1, of the fluid F1_1 or of the aqueousphase P1_1, in order to avoid decarboxylation reactions during thedehydration reaction, as described above.

The catalysts in the reactors R1 and R2 can be identical or different,it being possible for the level of the conversion and the selectivity inthe individual reaction vessels to be adjusted via the dwell time in theparticular catalyst bed, via the pressure, via the temperature and, inthe case of the second reaction vessel, via the reflux ratio. If ahomogeneous and usually liquid catalyst, in particular an inorganicacid, such as phosphoric acid, is employed, it is preferable for atleast the main amount of this catalyst to be added to the reactor R1.

The dehydration in the reactor R1, which is preferably a closed,pressure-resistant reactor, is, depending on the nature and amount ofthe catalyst used and depending on the pressure prevailing in thereaction vessel, preferably in a range of from 80 to 200° C.,particularly preferably in a range of from 120 to 160° C., while thepressure is preferably in a range of from 1 to 10 bar, particularlypreferably from 3 to 8 bar (abs.).

The fluid F1_1 obtained in this reactor R1 or the aqueous phase P1_1 isthen introduced into the reactor R2, which is constructed as adistillation or rectification column. In this context, the fluid F1_1 orthe aqueous phase P1_1 can be introduced either into the top region ofthe distillation column, preferably into a region of the upper third,particularly preferably in a region of the upper quarter and mostpreferably in a region of the upper quarter of the distillation orrectification column, or also in a side region of the distillation orrectification column, preferably in a region between the lower and theupper third, particularly preferably between the lower and the upperquarter of the distillation or rectification column.

If the fluid F1_1 or the aqueous phase P1_1 is introduced into the topregion of the distillation or rectification column, in this case thewater is preferably distilled off in countercurrent. If the fluid F1_1or the aqueous phase P1_1 is introduced in the side region of thedistillation or rectification column, it is preferable for thedistillation or rectification column to include a lower reaction region,in which the catalyst is located, and, adjacent to this reaction region,an upper distillation region which is free from catalyst, and for thefluid F1_1 or the aqueous phase P1_1 to be introduced into thedistillation column between the reaction region and the distillationregion.

The temperature in the reactor R2 is preferably in the same range as thetemperature in the reactor R1, while the pressure is lower, in order torender distillation of the water possible. The pressure of the reactorR2 is preferably lowered by 1 to 5 bar compared with the first reactorR1.

Preferably, an aqueous acrylic acid solution which contains no catalystconstituents (such a solution is obtained in the case of aheterogeneously catalysed dehydration), or an aqueous acrylic acidsolution which contains catalysts (such a solution is obtained in thecase of a homogeneously catalysed dehydration), as an aqueous phase P2,is obtained as the reaction mixture or as the fluid F2 which is obtainedafter the dehydration. If the organic phase obtained in a multiple phaseseparation has been employed as the fluid F1, an organic phasecontaining acrylic acid, as the fluid F2, is preferably obtained as thereaction mixture which is obtained after the dehydration, this organicphase optionally still containing catalyst constituents, depending onwhether a homogeneous or heterogeneous catalyst has been employed.

If an aqueous fermentation solution which optionally has been freed fromsolids and has optionally been reduced in its water content beforehand,has been employed as the fluid F1, it is preferable for the fluid F2obtained in process steps (a2) to be an aqueous phase P2 which has acomposition C3 comprising:

-   (C3_(—)1) 20 to 95 wt. %, preferably 30 to 90 wt. % and most    preferably 50 to 85 wt. % of acrylic acid, salts thereof or mixtures    thereof,-   (C3_(—)2) 0 to 5 wt. %, preferably 0.1 to 2.5 wt. % and most    preferably 0.5 to 1 wt. % of inorganic salts,-   (C3_(—)3) 0.1 to 30 wt. %, preferably 0.5 to 20 wt. % and most    preferably 1 to 10 wt. % of organic compounds which differ from    acrylic acid, in particular hydroxypropionic acids,-   (C3_(—)4) 0 to 50 wt. %, preferably 1 to 40 wt. %, preferably 5 to    20 wt. % and most preferably 1 to 10 wt. % of cells, and-   (C3_(—)5) 1 to 90 wt. %, preferably 50 to 80 wt. % and most    preferably 10 to 70 wt. % of water,    wherein the sum of components (C3_(—)1) to (C3_(—)5) is 100 wt. %.    Especially if the dehydration of the hydroxypropionic acid has been    carried out by means of the reactive distillation process described    above, it is preferable for the composition C3 which contains    acrylic acid and is obtained as the bottom product in the reactive    distillation to contain less than 25 wt. %, particularly preferably    less than 10 wt. % and most preferably less than 5 wt. % of water,    in each case based on the total weight of the composition C3.

It may furthermore be advantageous to purify the fluid F1 or the fluidF2 by an adsorption process, in particular by purification with acommercially available filter, in particular an active charcoal filter,before process step (a2) or (a3) is carried out.

If no solids, such as, for example, cells, have as yet been separatedoff, it may be helpful to separate off these solids by the filtrationprocesses described above, for example by ultrafiltration, beforecarrying out the crystallization in process step (a3).

In process step (a3) of the process according to the invention, theacrylic acid contained in the fluid F2 is now purified bycrystallization, preferably by a suspension crystallization or a layercrystallization, to give a purified phase, a suspension crystallizationcarried out continuously being particularly preferred. In one embodimentof the process according to the invention, the crystallization canalready be carried out with the water-rich aqueous phase P2 (or phaseF1_1) containing acrylic acid and hydroxypropionic acid, such as isobtained after the dehydration in the first reactor R1. In anotherembodiment of the process according to the invention, thecrystallization is carried out with the phase F1_2 which originates fromthe further reactor and has a lower water content compared with thephase P2. The phase P2 often contains more than 30 wt. %, preferablymore than 50 wt. % and particularly preferably more than 70 wt. % ofwater in addition to acrylic acid and hydroxypropionic acid. On theother hand, the phase F1_2 often contains less than 30 wt. %, preferablyless than 20 wt. % and particularly preferably less than 10 wt. %, inaddition to acrylic acid and hydroxypropionic acid.

In the case of suspension crystallization, the crystals can be separatedoff from the mother liquor by a washing column. For successful operationof a washing column, it is advantageous if the crystals to be washedhave a sufficient hardness and have a particular narrow sizedistribution, in order to ensure a corresponding porosity and stabilityof the packed or non-packed filter bed formed.

The suspension crystallization can advantageously be realized in astirred tank crystallizer, scraped surface crystallizer, cooling diskcrystallizer, crystallizing screw, drum crystallizer, tube bundlecrystallizer or the like. In particular, the crystallization variantsmentioned in WO-A-99/14181 can be used for the purpose mentioned. Thosecrystallizers which can be operated continuously in particular are inturn of particular advantage here. These are preferably the cooling diskcrystallizers or the scraped surface cooler (dissertation by Poschmann,p. 14). A scraped surface cooler is very particularly preferablyemployed for the crystallization.

In principle, any washing column which allows the continuous procedureof the purification according to the invention can be employed for theprocess according to the invention. In a conventional embodiment, thesuspension is introduced into a hydraulic washing column in the upperpart of the column. The mother liquor is removed from the washing columnvia a filter, a densely packed crystal bed forming. The mother liquorflows through the crystal bed in the direction of the base of the columnand forces this downwards due to the flow resistance. At the base of thecolumn is a moving, preferably rotating scraping device or scraper,which generates a suspension again from the densely packed crystal bedand the wash melt introduced at the lower part of the washing column.This suspension is preferably pumped off through a melting unit,preferably a heat exchanger, and melted. A part of the melt can servee.g. as the washing melt; this is then pumped back into the column andpreferably washes out the crystal bed migrating in the oppositedirection, i.e. the crystallized acrylic acid is washed incountercurrent by the recycled acrylic acid. Against the background ofincreasing the yield, recycling of the mother liquor which has beenseparated off is particularly advantageous. The washing melt on the onehand effects washing of the crystals, and on the other hand the melt atleast partly crystallizes out on the crystals. The crystallizationenthalpy liberated heats the crystal bed in the wash region of thecolumn. A purification effect analogous to sweating of the crystals isthereby achieved.

A purification is therefore effected on the one hand by the washing ofthe surface of the acrylic acid with molten—and therefore alreadypurified acrylic acid, and on the other hand healing or exudation ofimpurities is achieved by the crystallization of the molten purifiedacrylic acid on the acrylic acid crystals already present. This allows aparticularly highly pure preparation of acrylic acid.

However, the washing of the crystals obtained by suspensioncrystallization can in principle also be carried out in a manner otherthan countercurrent washing in a washing column. Thus, for example, thecrystals can be washed on a belt filter after they have been separatedoff from the mother liquor by means of a suitable separating device, forexample a filter.

According to a further embodiment of the process, the aqueous phase P2,preferably the phase F1_2, is cooled to a maximum to the temperature Tof the triple eutectic point of a composition of acrylic acid, water andhydroxypropionic acid, preferably down to a temperature which is at most6 Kelvin, particularly preferably at most 3 Kelvin and most preferablyat most 1 Kelvin above the temperature of the triple eutectic point.

Preferably, in the purification by means of crystallization a firstcrystal phase is obtained, containing:

-   -   at least 30 wt. % of acrylic acid, preferably at least 40 wt. %        and most preferably at least 50 wt. % of acrylic acid,    -   at least 30 wt. %, preferably at least 40 wt. % and most        preferably at least 49 wt. % of water, and    -   at most 10 wt. %, preferably at most 5 wt. % and most preferably        at most 1 wt. % of hydroxypropionic acid.

The purification of the acrylic acid from the fluid F2, in particularfrom the aqueous phase P2, by means of crystallization can be carriedout in one, two, three or more stages. In a two-stage crystallization,the crystals obtained in the first crystallization stage are separatedoff from the mother liquor which remains, melted and crystallized againin a second crystallization stage.

Especially if a composition C3 which, in addition to acrylic acid,organic compounds which differ from acrylic acid, such as, in particularhydroxypropionic acid which has not been dehydrated in process step(a2), and optionally inorganic salts, contains more than 5 wt. %,particularly preferably more than 10 wt. % and most preferably more than25 wt. % of water is employed as the fluid F2, preferably as the aqueousphase P2, according to a particular embodiment of the process accordingto the invention it is preferable for the crystallization in processstep (a3) to be configured as an at least two-stage and preferably atleast three-stage crystallization. In the case of the at least two-stagecrystallization, it is preferable if in at least two preferablysuccessive stages the particular eutectic of the main components of sucha stage, which as a rule are contained in the particular feed with 1 andmore wt. %, is approached by varying the crystallization conditions,preferably lowering the temperature. If crystallization is carried outin three and more stages, it is preferably if before the stage in whichthe eutectic of the particular main components of the feed, preferablywater and acrylic acid, is not reached by varying the crystallizationconditions, preferably lowering the temperature, at least one,preferably at least two stages in which the particular eutectic of themain components is approached by varying the crystallization conditions,preferably lowering the temperature, are carried out. In the abovestages it is preferable for a cooling rate in a range of from 0.01 to 10K/min, preferably in a range of from 0.05 K/min to 5 K/min andparticularly preferably in a range of from 0.1 to 1 K/min to be used.

It is thus furthermore preferable for process step (a3) to comprise thefollowing part steps, which preferably follow one another directly:

-   (a3_1) crystallization, preferably suspension or layer    crystallization, most preferably suspension crystallization, of the    fluid F2, in particular of the aqueous phase P2, in a first    crystallization stage to give a crystal phase K1 and a mother liquor    M1, wherein the crystal phase K1 comprises:    -   5 to 60 wt. %, particularly preferably 10 to 55 wt. % and most        preferably 15 to 50 wt. % of acrylic acid,    -   39.9 to 95 wt. %, particularly preferably 44.6 to 90 wt. % and        most preferably 80 to 99.5 wt. % of water, and    -   0.1 to 10 wt. %, particularly preferably 0.4 to 8 wt. % and most        preferably 1 to 6 wt. % of by-products which differ from water        and acrylic acid,        and wherein the sum of the amounts by weight of acrylic acid,        water and by-products is 100 wt. %,-   (a3_2) separating off of the crystal phase K1 from the mother liquor    M1, preferably by means of a washing column, the crystals preferably    being subjected to a crystal washing in the nature and manner    described above,-   (a3_3) melting of the crystal phase K1 from the first    crystallization stage,-   (a3_4) renewed crystallization, preferably suspension or layer    crystallization, most preferably suspension crystallization, of the    molten crystal phase in a second crystallization stage to give a    crystal phase CR2 and a mother liquor M2, wherein the crystal phase    K2 comprises:    -   8 to 35 wt. %, particularly preferably 1 to 28 wt. % and most        preferably 0.4 to 19.75 wt. % of acrylic acid,    -   at least 60 wt. %, particularly preferably at least 70 wt. % and        most preferably 80 to 99.5 wt. % of water, and    -   a maximum of 5 wt. %, particularly preferably a maximum of 2 wt.        % and most preferably 0.1 to 0.25 wt. % of by-products which        differ from water and acrylic acid,        wherein the sum of the amounts by weight of acrylic acid, water        and by-products is 100 wt. %,-   (a3_(—)5) separating off of the crystal phase K2 from the mother    liquor M2, preferably by means of a washing column,-   (a3_(—)6) crystallization, preferably suspension or layer    crystallization, most preferably suspension crystallization, of the    mother liquor M2 in a third crystallization stage to give a crystal    phase K3 and a mother liquor M3, wherein the crystal phase K3    comprises:    -   at least 40 wt. %, particularly preferably at least 50 wt. % and        most preferably 55 to 70 wt. % of acrylic acid,    -   a maximum of 70 wt. %, particularly preferably a maximum of 57.5        wt. % and most preferably 45 to 57.5 wt. % of water, and    -   a maximum of 5 wt. %, particularly preferably a maximum of 2.5        wt. % and most preferably 0 to 1.5 wt. % of by-products which        differ from water and acrylic acid,        wherein the sum of the amounts by weight of acrylic acid, water        and by-products is 100 wt. %, and-   (a3_(—)7) separating off of the crystal phase K3 from the mother    liquor M3, preferably by means of a washing column, the crystals    preferably being subjected to a crystal washing in the nature and    manner described above, a purified phase containing acrylic acid    crystals finally being obtained.

In the particular embodiment of the process according to the inventiondescribed above with a three-stage crystallization stage in process step(a3), it may be particularly advantageous to feed the mother liquorobtained in the first crystallization stage, which still containsrelatively large amounts of non-dehydrated hydroxypropionic acid, backinto process step (a2) for the purpose of a dehydration which is ascomplete as possible.

Embodiments of suspension crystallization with subsequent washing of thecrystals in a hydraulic or mechanical washing column are to be found inthe book “Melt crystallization technology” by G. F. Arkenbout, TechnomicPublishing Co. Inc., Lancaster-Basle (1995), p. 265 to 288 and thearticle directed at the Niro freeze concentration for preconcentrationof waste water in Chemie-Ingenieurtechnik (72) (1Q/2000), 1231 to 1233.

A washing liquid familiar to the person skilled in the art can be usedas the washing liquid, depending on the intended use (for aqueoussolutions e.g. water). As already indicated, a part amount of the moltencrystals of the crystallized acrylic acid, the crystallized water or thecrystallized mixture of water and acrylic acid very particularlypreferably serves for washing thereof. This measure on the one handensures that no further substance has to be introduced into the systemfor the production of highly pure products, and on the other hand themolten crystals also serve to push back the mother liquor front in thewashing column and at the same time have a purifying effect on thecrystals, analogously to sweating. In this context, no loss of producttakes place since the washing liquid crystallizes out in turn on thecrystals to be washed and is recovered in the product in this way (e.g.brochure of Niro Process Technology B.V. Crystallization and wash columnseparations set new standards in purity of chemical compounds).

In an alternative embodiment, the formation of the crystals in at leastone of process steps (a3_1), (a3_4) and (a3_6) is carried out in alayer. A layer crystallization is carried out in accordance with theprocess of Sulzer AG, Switzerland (http://www.sulzerchemtech.com). Asuitable layer crystallizer and the procedure during layercrystallization is described, for example, in WO-A-00/45928, which isintroduced herewith as reference and the disclosure content of whichwith respect to layer crystallization forms a part of the disclosure ofthe present invention.

In the end, an acrylic acid having a purity of at least 50 wt. %, inparticular at least 70 wt. %, preferably at least 90 wt. % can beprepared by the process according to the invention for the preparationof acrylic acid.

The process according to the invention for the preparation ofpolyacrylates envisages polymerization, preferably free radicalpolymerization, of an acrylic acid obtainable from the process accordingto the invention.

The free radical polymerization of the acrylic acid is carried out bythe polymerization process known to the person skilled in the art. Ifthe polymers are crosslinked, partly neutralized polyacrylates,reference is made to the 3rd chapter (page 69 et seq.) in “ModernSuperabsorbent Polymer Technology”, F. L. Buchholz and A. T. Graham(editors), Wiley-VCH, New York, 1998 with respect to the preciseprocedure.

A contribution towards achieving the abovementioned objects isfurthermore made by the acrylic acid obtainable by the process accordingto the invention for the preparation of acrylic acid, and thepolyacrylates obtainable by the process according to the invention forthe preparation of polyacrylates.

A further contribution towards achieving the abovementioned objects ismade by chemical products containing the polyacrylates according to theinvention. Preferred chemical products are, in particular, foams, shapedarticles, fibers, foils, films, cables, sealing materials,liquid-absorbing hygiene articles, in particular diapers and sanitarynapkins, carriers for plant or fungal growth-regulating agents or plantprotection active compounds, additives for building materials, packagingmaterials or soil additives.

The use of the polyacrylates according to the invention in chemicalproducts, preferably in the abovementioned chemical products, inparticular in hygiene articles, such as diapers or sanitary napkins, andthe use of the superabsorber particles as carriers for plant or fungalgrowth-regulating agents or plant protection active compounds also makea contribution towards achieving the abovementioned objects. For the useas a carrier for plant or fungal growth-regulating agents or plantprotection active compounds it is preferable for it to be possible forthe plant or fungal growth-regulating agents or plant protection activecompounds to be released over a period of time controlled by thecarrier.

The device according to the invention for the preparation of acrylicacid comprises the following units connected to one another byfluid-carrying lines:

-   a synthesizing unit for the preparation of hydroxypropionic acid    from a biological material to give a fluid F1 containing    hydroxypropionic acid, in particular an aqueous phase P1,-   a dehydration stage for the hydroxypropionic acid to give a fluid F2    containing acrylic acid, in particular an aqueous phase P2, and-   a purification unit for the fluid F2 containing acrylic acid by a    suspension crystallization or a layer crystallization to give a    purified phase.

According to the invention, “by fluid-carrying lines” is understood asmeaning that gases or liquids, suspension included, or mixtures thereofare led through corresponding lines. Pipelines, pumps or the like can beemployed in particular for this.

Hydroxypropionic acid is synthesized from a regenerable raw materialwith the aid of the synthesizing unit, and is subsequently convertedinto acrylic acid by means of the dehydration stage. The acrylic acid isthen purified in the context of a layer crystallization or a suspensioncrystallization. In addition to a conventional bioreactor, the deviceaccording to the invention can also include a sterilization unit, inwhich the microorganisms contained in the fermentation broth can bekilled after the fermentation has ended, a filtration unit, in whichsolids contained in the fermentation broth can be separated off byfiltration, preferably by ultrafiltration, a protonation unit, in whichat least a part of the hydroxypropionic acid optionally present as asalt can be converted into its acid form, and/or a dewatering unit, inwhich at least a part of the water can be separated off In this context,these units can be located both between the synthesizing unit and thedehydration stage and between the dehydration stage and the purificationunit, at least the sterilization unit and the filtration unit preferablybeing located between the synthesizing unit and the dehydration stage.

According to a preferred embodiment of the device according to theinvention, the dehydration stage is constructed as a pressure reactor,it furthermore being particularly preferable for the dehydration stageto include a distillation or rectification device which contains one ofthe dehydration catalysts described above. The reactive distillationdescribed in connection with the process according to the invention forthe preparation of acrylic acid can be carried out by means of such adehydration stage. All the known forms of construction of distillationcolumns, in particular packed columns and plate columns, are suitable asthe device for this purpose.

In this context it may be particularly advantageous if the dehydrationstage has at least two dehydration reactors R1 and R2 which areconnected to one another by fluid-carrying lines and which all containone of the dehydration catalysts described above, one of which can beloaded with a pressure Π1 and the other with a pressure Π2, where thetwo pressures differ from one and the pressure Π2 is preferably lowerthan Π2. The preferred pressure differences are those pressuredifferences which have already been described above in connection withthe process according to the invention. Preferably, the reactor R2 isconstructed as a distillation or rectification device and the reactor R1is not. This distillation or rectification device is connected to thepurification unit such that the bottom product obtained in thedistillation or rectification can be transferred into the purificationunit.

At least a part of the hydroxypropionic acid is converted into acrylicacid by means of the dehydration reactor R1, into which the fermentationsolution which has preferably been freed from solids and optionally atleast partly dewatered is introduced. In this context, the firstdehydration reactor R1 is preferably connected to the second dehydrationreactor R2 such that the aqueous solution which containshydroxypropionic acid, acrylic acid and water and is obtained in thefirst reactor R1 is introduced into the top region of the secondreactor, preferably into a region of the upper third, particularlypreferably in a region of the upper quarter and most preferably in aregion of the upper quarter of the reactor R2, or also in a side regionof the reactor R2, preferably in a region between the lower and theupper third, particularly preferably between the lower and the upperquarter of the reactor R2. The circumstance that for a reactivedistillation the low liquid content of the packed columns employed forthe rectification is a disadvantage, especially in the case of reactionswith a finite rate of reaction, can be taken into account by means ofthe reactor R1. In order to achieve the desired conversions in slowreactions, a long dwell time of the liquid in the apparatus must beensured. As an aid, in addition to the column one or more external tanks(reactor 1, further external tanks (reactors, R1′, R1″ etc.) canoptionally also be present) in which a major part of the actual reactiontakes place can be installed. Recycling of the liquid from these tanksinto the column in each case requires division of the packing and anelaborate distribution of liquid, so that plate columns withcomparatively high liquid contents are advantageously employed for thereactive distillation. Plate columns have a high liquid content, andindeed both in the two-phase layer on the plate and in the downcomers.Both portions of the liquid content can be modified in a targeted mannerand adapted to the reactive distillation by construction measures, i.e.by the design of the plates or the height of the weirs. The followingaspects play an important role when designing suitable built-incomponents for a reactive plate column: (i) realization of very highliquid contents, i.e. dwell times; (ii) distribution of the hold-up overthe column height as desired; (iii) suitability both for homogeneous andfor heterogeneous catalysis; (iv) easy replacement of the catalyst; (v)simple scale-up from the laboratory to large-scale industrial columns.

The fluid F2 containing the acrylic acid can be purified by a suspensioncrystallization or a layer crystallization by means of the purificationunit to give a purified phase.

According to a particularly preferred embodiment of the device accordingto the invention, the purification unit includes the followingconstituents connected to one another by fluid-carrying lines:

-   (δ1) a first crystallization region, a second crystallization    region, a third crystallization region, a first separating region, a    second separating region, a third separating region, at least one    melting unit and at least six guide lines,-   (δ2) the first crystallization region is connected to the first    separating region via a first guide line,-   (δ3) the first separating region is connected to the at least one    melting unit via a second guide line,-   (δ4) the at least one melting unit is connected to the second    crystallization region via a third guide line,-   (δ5) the second crystallization region is connected to the second    separating region via a fourth guide line,-   (δ6) the second separating region is connected to the third    crystallization region via a fifth guide line,-   (δ7) the third crystallization region is connected to the third    separating region via a sixth guide line.

In this context it is preferable for the dehydration stage preferably tobe connected directly to the first crystallization region byfluid-carrying lines, in the case of a reactive distillation orrectification column in the dehydration stage these columns preferablybeing connected to the first crystallization unit such that the bottomproduct obtained can be transferred into the crystallization unit. Inone embodiment according to the invention, this crystallization deviceaccording to the invention having several successive crystallizationregions can follow the first reactor R1. Very pure aqueous acrylic acidsolutions can be obtained in this way, so that a polymerization devicein which this acrylic acid solution is polymerized can follow thecrystallization device directly.

If suspension crystallizers are employed as crystallization regions andwashing columns are employed as separating regions, with this particularembodiment of the device according to the invention the fluid F2 whichcontains acrylic acid and is obtained in the dehydration stage can becrystallized in the first crystallization region to give crystals ofwater and acrylic acid. These can then be washed in a washing column(first separating unit) and separated off from the mother liquor whichremains and which above all still contains hydroxypropionic acid. Thecrystals separated off, which above all contain water and acrylic acid,can then be melted in the at least one melting unit and thencrystallized again in the second crystallization region. The crystalswhich are obtained in this second crystallization region and above allare based on water can be separated off from the mother liquor in afurther washing column (second separating unit). The mother liquor whichremains can then be crystallized in a third crystallization region togive crystals which are essentially based on acrylic acid, it beingpossible for these crystals then to be washed in a further washingcolumn (third separating unit) and separated off from the mother liquorwhich remains.

According to an embodiment, which is furthermore advantageous, of theparticularly preferred embodiment described above for the deviceaccording to the invention in which three crystallization regions arepresent, the first separating unit and the third separating unit areconnected to a second and, respectively, a third melting unit byfluid-carrying lines. The crystals which have been separated off fromthe mother liquor in the first and third separating unit can be at leastpartly melted with this second and, respectively, third melting unit,and the crystal melts obtained in this way can be employed for thecountercurrent washing of the crystal phases present in the particularseparating units, as is described, for example, in DE-A-101 49 353.

Because of the purification method, the acrylic acid prepared in thisway is treated particularly gently, as a result of which its quality isimproved. The purification device is capable of obtaining very pureacrylic acid having purities of more than 60 per cent by weight, inparticular of more than 99.5 per cent by weight, from a comparativelycontaminated acrylic acid stream, which above all contains water andhydroxypropionic acid as by-products, of about 12 per cent by weight.According to the invention, it is possible to purify efficiently anacrylic acid stream with 50 per cent by weight to 95 per cent by weightof acrylic acid, preferably 75 to 90 per cent by weight of acrylic acid.

The effective purification enables further thermal treatment methods, inparticular a distillation or evaporation, to be reduced, so thatexposure of the acrylic acid and the acrylic acid formed therefrom toheat is reduced. As a result, the quality of the acrylic acid and of theacrylic acid is improved.

To further increase the purity of the acrylic acid, the device unit hasa separate purification device. This separate purification device can beemployed for further purification of the end product, in particular forfurther purification of the acrylic acid leaving the melting unit.

To increase the yield, it is particularly preferable for the firstseparating region to be connected to the dehydration reactor, so thatthe mother liquor obtained in the separating region can be introducedinto the dehydration stage again for the purpose of a conversion of thehydroxypropionic acid which is as complete as possible.

In the process according to the invention for the purification ofacrylic acid, temperatures in the range of from −20 to +20° C.,preferably from −10 to +13° C. under a pressure of from 1 to 10 barprevail in the separating regions. It is preferable for a lowertemperature and a lower pressure to prevail in the lower region of theseparating regions than in the upper region of the separating regions.Preferably, −20 to <12° C. under a pressure of 1 to 2 bar prevail in thelower region of the separating regions. In the upper region of theseparating regions, a temperature of at least 12° C. and a pressure offrom 1 to 10 bar, preferably 3 to 7 bar prevail. Advantageously,temperatures in the range of from −20 to 20° C., preferably from −12 to13° C. under a pressure of from 0.5 to 10 bar, preferably from 0.8 to 2bar prevail in the crystallization regions. A temperature in the rangeof from 10 to 50° C., preferably from 11 to 10° C. under a pressure offrom 1 to 10 bar, preferably 3 to 7 bar can prevail in the at least onemelting unit.

In the guide lines, temperature and pressure conditions which allow areliable and trouble-free transportation of the acrylic acid and thesubstances which optionally accompany this in these guide lines prevail.

The device allows a relatively contaminated acrylic acid, whichoptionally still contains large amounts of water and hydroxypropionicacid, to be used as the starting material, as a result of which thepreliminary outlay for a distillation of the acrylic acid originatingfrom the synthesis becomes lower. In particular, the exposure of theacrylic acid to heat, which can lead to undesirable polymerization, orto a premature formation of acrylic acid, undesirable polymerizationresulting therefrom, is therefore lowered.

In a modification of the invention, in particular in combination withthe device according to the invention for the preparation of acrylicacid by means of a layer crystallization or a suspensioncrystallization, a device according to the invention for the preparationof acrylic acid comprises the following units connected to one anotherby fluid-carrying lines:

-   a synthesizing unit for 3-hydroxypropionic acid from a biological    material to give a fluid, in particular aqueous, phase containing    3-hydroxypropionic acid,-   a dehydration stage for the 3-hydroxypropionic acid to give a fluid,    in particular aqueous, solution containing acrylic acid and-   at least one of the following separating devices (S1) to (S5) as a    dewatering unit:    -   (S1) reverse osmosis device,    -   (S2) electroosmosis device,    -   (S3) azeotropic distillation device,    -   (S4) electrodialysis device,    -   (S5) multiple phase separation device.

This modification can be employed in particular as a preliminary stagebefore the purification unit. It is also preferable in particular for atleast one of the separating devices (S1) to (S5) to be inserted betweenprocess steps (a1) and (a3), in particular between (a1) and (a2), inorder to achieve a pre-purification effect.

The reverse osmosis device is suitable for carrying out a reverseosmosis; the electroosmosis device is suitable for carrying out anelectroosmosis, the azeotropic distillation device is suitable forcarrying out an azeotropic distillation; the electrodialysis device issuitable for carrying out an electrodialysis; the multiple phaseseparation device is suitable for carrying out a multiple phaseseparation, in particular the abovementioned “salt splitting”.

In a specific further development, the device comprises a furtherseparating device, in particular filter, for separating off solids, inparticular particles, such as microorganisms, cells or parts thereof,from the fluid phase, from the fluid solution or from both. The furtherseparating device for separating off solids is preferably located, asalready described above, between the synthesizing unit and thedehydration stage. The separate separating device for separating offsolids can also be arranged between the dehydration stage and thepurification unit or the separating devices (S1) to (S5).

The device furthermore advantageously comprises a protonation means forthe hydroxypropionic acid in the fluid phase. The protonation can becarried out in particular by addition of an acid, for examplehydrochloric acid, or ammonium chloride. The protonation means can alsobe an ion exchanger, with which the hydroxypropionic acid is convertedinto the free acid.

The protonation means is arranged in particular downstream of thesynthesizing unit, i.e. in particular between the synthesizing stage andthe dehydration stage or between the dehydration stage and one of theseparating devices (S1) to (S5) functioning as a dewatering unit or thepurification unit. The protonation means can also be arranged directlyat the dehydration stage or purification unit or separating device (S1)to (S5) and can be a feed for a reagent which effects protonation of thehydroxypropionic acid, such as e.g. an acid.

The device can have adsorption means or absorption means, in particularan active charcoal filter, for further purification of the fluid phaseor of the fluid solution, or both. Fine particles in the suspension,such as e.g. biological residues, can be removed from the fluid phase orsolutions with the aid of the ad- or absorption means, it being possiblefor this absorption means to be located both between the synthesizingunit and the dehydration stage and between the dehydration stage and thepurification unit.

The device according to the invention for the preparation ofpolyacrylates comprises the device according to the invention for thepreparation of acrylic acid and a polymerization reactor for thepolymerization of acrylic acid, in particular a polymerization reactorfor the free radical polymerization of acrylic acid.

In a preferred embodiment of the process according to the invention forthe preparation of acrylic acid, the device described above is employed.

In the process according to the invention for the preparation ofpolyacrylates, an acrylic acid obtained by the process according to theinvention is polymerized, this polymerization preferably being carriedout in the presence of crosslinking agents. If the polymers arecrosslinked, partly neutralized polyacrylates (so-calledsuperabsorbers), reference is made to the 3rd chapter (page 69 et seq.)in “Modern Superabsorbent Polymer Technology”, F. L. Buchholz and A. T.Graham (editors), Wiley-VCH, New York, 1998 with respect to the preciseprocedure.

The polyacrylates according to the invention are preferablycharacterized by a sustainability factor of at least 10, preferably atleast 20, particularly preferably at least 50, moreover preferably atleast 75, in addition preferably at least 85 and furthermore preferablyat least 95. In this context, the sustainability factor indicates theproportion to which the water-absorbing polymer structure is based onnon-fossil regenerating organic material. If the sustainability factoris 100, the polymer structure comprises entirely substances based onnon-fossil regenerating organic materials.

The polyacrylates obtained by the process according to the invention forthe preparation of polyacrylates are preferably a water-absorbingpolymer structure. This water-absorbing polymer structure preferably hasat least one of the following properties:

-   (β1) a CRC value (CRC=centrifugation retention capacity), determined    in accordance with ERT 441.2-02 (ERT=edana recommended test method),    of at least 20 g/g, preferably at least 25 g/g and most preferably    at least 30 g/g, a CRC value of 60 g/g, preferably of 50 g/g not    being exceeded;-   (β2) an absorption against a pressure of 0.7 psi (AAP), determined    in accordance with ERT 442.2-02, of at least 16 g/g, preferably at    least 18 g/g and most preferably at least 20 g/g, a value of 50 g/g,    preferably of 40 g/g not being exceeded.

The fibers, films, molding compositions, textile and leatherauxiliaries, flocculating agents, coatings or lacquers according to theinvention are based on acrylic acid which has been obtained by theprocess according to the invention or on derivatives thereof, such as,for example, on esters of this acrylic acid, while according to the useaccording to the invention of the acrylic acid obtained by the processaccording to the invention for the preparation of acrylic acid, this orits derivatives is or are preferably employed in fibers, films, moldingcompositions, textile and leather auxiliaries, flocculating agents,coatings or lacquers.

Advantageous details and embodiments, which can in each case be used, orcombined with one another as desired, are explained further with the aidof the following drawings, which are not intended to limit the inventionbut merely to illustrate it by way of example.

FIG. 1 illustrates the process flow according to the invention and thedevice according to the invention. Via the feed 1 for biomass, such ase.g. rape or maize, this can be introduced into the synthesizing unit 2,the synthesizing unit 2 preferably being a bioreactor. In thesynthesizing unit 2 the biomass is converted to form an aqueous phase P1containing hydroxypropionic acids, preferably 2- or 3-hydroxypropionicacids, most preferably 3-hydroxypropionic acid. This aqueous phase P1can then be fed via an outlet 3 to a killing unit, which is, forexample, an autoclave or another sterilization device known to theperson skilled in the art. The aqueous phase P1 treated in this way canthen be fed via the outlet 5 to a filtration unit 6, in which theaqueous phase P1 is freed from solids, such as, for example,microorganisms. The solids separated off can be removed via a discharge7. If the microorganisms contained in the aqueous phase P1 are not to bekilled and are to be used for a further synthesis cycle, the solidsseparated off in the filtration unit 6 can also be at least partly fedback into the synthesizing unit (not shown in FIG. 1). (The units 111,112, 113 and 114 can also be arranged individually or in combinationbetween the synthesizing unit 2 and the dehydration stage 9, not shownin FIG. 1).

The aqueous phase P1 which has been freed from solids then enters, viathe outlet 8, into the dehydration stage 9, in which thehydroxypropionic acid is converted into acrylic acid, water being splitoff The aqueous phase P2 containing acrylic acid and optionallyappropriate unreacted hydroxypropionic acid can then be further treatedin further device constituents via the outlet 10. These further deviceconstituents can be, for example, a further filtration unit 111, aprotonation means 112, an adsorption means 113 or a dewatering unit 114,for example a reverse osmosis device, an electroosmosis device, anazeotropic distillation device, an electrodialysis device or a multiplephase separation device. These device units can be arranged individuallyor in combination in series between the dehydration stage 9 and thepurification unit 13. It may furthermore be advantageous to arrange oneor more of these further device constituents 11 between the synthesizingunit 2 and the dehydration stage 9, particularly preferably between thefiltration unit 6 and the dehydration stage 9, instead of or in additionto its arrangement between the dehydration stage 9 and the purificationunit 13 (not shown in FIG. 1).

FIG. 2 illustrates a preferred embodiment of the process flow accordingto the invention and of the device according to the invention, in whichthe dehydration is carried out by means of reactive distillation. Viathe outlet 8, via which the aqueous phase P1 which has been freed fromsolids in the filtration unit 6 is removed, this aqueous phase P1enters, usually via a membrane 90, into a first reactor 91, whichcontains a catalyst which promotes the dehydration and in which at leasta part of the hydroxypropionic acid is converted into acrylic acid. Anaqueous phase P2 containing acrylic acid, water, unreactedhydroxypropionic acid and optionally further by-products is thentransferred via a discharge line 92 into a second reactor 93, which is,for example, a distillation or rectification column which likewisecontains a catalyst which promotes the dehydration. In this secondreactor 93 the hydroxypropionic acid which has not yet reacted is nowconverted further into acrylic acid, and at the same time the watercontained in the aqueous phase is separated off via the discharge 94(reactive distillation). The bottom product 95 which is obtained in thesecond reactor 93 and contains the acrylic acid can then be fed via thedischarge line 10 to the further device constituents 11 or directly tothe purification unit 14 (see FIG. 1). Both the first reactor and thesecond reactor can be charged with carbon dioxide via a feed line 96, inorder to prevent decarboxylation reactions during the dehydration.

FIG. 3 likewise shows a preferred embodiment of the process flowaccording to the invention, wherein in this embodiment the purificationis carried out in a three-stage crystallization. The aqueous phase P2treated in the further device constituents 11 or the aqueous phase P2obtained directly from the dehydration stage 9 enters via the feed 12into a first crystallization device 131, which is preferably asuspension crystallizer. However, the use of other crystallizationdevices, such as, for example, a layer crystallizer, is alsoconceivable. The aqueous phase P2 containing acrylic acid is preferablycooled in the first crystallization device 131 to the extent that waterand acrylic acid crystallize out. If a suspension crystallizer isemployed as the first crystallization device, the crystal suspensionobtained in this way is fed via the discharge line 132 to a firstseparating region 133, which is preferably a washing column and in whichthe crystals are separated off from the mother liquor which remains. Ifa layer crystallizer is employed as the first crystallization device,the use of a separating unit is not essential (not shown in FIG. 3),since in this case it is not necessary to separate off the crystals fromthe mother liquor, for example by means of a washing column, due to theimmobilizing of the crystals on the surfaces of the layer crystallizer.The crystals separated off from the mother liquor in the firstseparating unit can be at least partly melted by means of a melting unit134 for the purpose of further increasing the purity, and the moltencrystals can be employed, by means of a conveying element 135, forexample by means of a pump, for washing the crystals in countercurrent,as is described in DE-A-101 49 353. If a layer crystallizer has beenemployed as the first crystallization device 131, it may also beadvantageous here to at least partly melt the crystals and to employ thecrystal melt obtained for washing the crystals in countercurrent.

The crystals which are obtained in the first separating region after themother liquor has been separated off and essentially comprise water andacrylic acid can then be fed via the discharge line 136 to a meltingunit 137, in which the crystals are melted as completely as possible.The mother liquor which remains and which can still contain relativelylarge amounts of unreacted hydroxypropionic acid can advantageously befed back via the discharge line 1319 into the dehydration stage 9. Thecrystals melted in this way are then fed via the discharge line 138 to asecond crystallization device 139, in which the temperature is loweredto the extent that crystals essentially comprising water are formed.This second crystallization device is likewise preferably a suspensioncrystallizer, but the use of a layer crystallizer is also conceivablehere. If a suspension crystallizer has been employed as the secondcrystallization device 139, the crystal suspension obtained in thesecond crystallization device 139 can then be fed by means of thedischarge line 1310 to a second separating region 132, in which thecrystals essentially based on water are separated off from the motherliquor essentially based on acrylic acid and water via a discharge 1311.Here also, in the case where a layer crystallizer is employed as thecrystallization device, the use of a separate separating unit, such as,for example, a washing column, is not essential.

The mother liquor separated off from the crystals in the secondseparating region 132 can then be fed via the discharge line 1313 to thethird crystallization device 1314, in which the temperature is loweredto the extent that crystals essentially based on acrylic acid areformed. This third crystallization device 1314 can likewise be asuspension crystallizer or a layer crystallizer. If a suspensioncrystallizer has been employed in the third crystallization device, thecrystal suspension obtained in the third crystallization device can befed via the discharge line 1315 to the third separating region 1316, inwhich the acrylic acid crystals are separated off from the motherliquor. As is also the case with the first crystallization stage, it maybe advantageous to melt at least a part of the crystals in the thirdseparating region by means of a melting unit 1317 and then to employ themelt, via a conveying element 1318, such as, for example, a pump, forwashing the crystals in countercurrent. The acrylic acid crystalsobtained in the third separating region 1316 can then be removed via adischarge line 14. If a layer crystallizer has been employed as thethird crystallization device 1314, it may also be advantageous here toat least partly melt the crystals and to employ the molten crystals forwashing the crystals.

The present invention is now explained in more detail with the aid ofnon-limiting examples.

EXAMPLES Preparation Example 1

Preparation of a test composition 1 simulating a fermentation broth andcomprising:

1,000 ml of water

100 g of 3-hydroxypropionic acid (10 wt. %)

9 g of baker's yeast

1 g of glucose as organic material,

a pH of from 6.5 to 7.5 having been established by means of ammonia.

Preparation Example 2

Preparation of a test composition 2 simulating a purified fermentationbroth and comprising:

1,000 ml of water

100 g of 3-hydroxypropionic acid (10 wt. %)

Dehydration 1:

In a device shown in FIG. 2, the solution obtained in PreparationExample 1 is first freed from the yeast by a membrane 90 (commerciallyobtainable from Amafilter GmbH) and heated in a first pressure reactor91 (flow-through high-grade steel reactor tube having an internaldiameter of 17 mm and a length of 50 cm) at a temperature of 140° C.under a pressure of 8 bar over a dwell time of 8 minutes in the presenceof phosphoric acid in a concentration of 1 part of phosphoric acid per5,000 parts of educt stream.

The reaction mixture obtained in this way, which above all containswater, unreacted 3-hydroxypropionic acid and acrylic acid, wassubsequently introduced into a reaction column 93 (autoclave withattached distillation column) and likewise heated at a temperature of140° C. under a pressure of 3.7 bar over a dwell time of 16 minutes inthe presence of the phosphoric acid initially employed, as a catalyst.During this procedure, the water contained in the reaction mixture wasseparated off over the top and a bottom product rich in acrylic acid wasobtained.

Dehydration 2:

Dehydration 1 was repeated here with the following differences: Thereaction mixture obtained from the pressure reactor 91, which above allcontains water, unreacted 3-hydroxypropionic acid and acrylic acid, wassubsequently introduced into a reaction column 93 (autoclave withattached distillation column) and likewise heated at a temperature of120° C. under a pressure of 500 bar over a dwell time of 45 minutes inthe presence of the phosphoric acid initially employed, as a catalyst.During this procedure, the water contained in the reaction mixture wasseparated off over the top and a bottom product rich in acrylic acid wasobtained.

Dehydration 3:

Dehydration 1 was repeated here with the following differences: Insteadof phosphoric acid, CO₂ was fed into the reactor 91 to saturation.

Dehydration 4:

Dehydration 1 was repeated here, with the difference that testcomposition 2 was employed.

Dehydration 5:

Dehydration 2 was repeated here, with the difference that testcomposition 2 was employed.

Dehydration 6:

Dehydration 3 was repeated here, with the difference that testcomposition 2 was employed.

The bottom products of the above dehydrations have the compositionsshown in Table 1.

TABLE 1 Dehydration 1 2 3 4 5 6 Water  3%  4%  4% 6   5%  6%3-Hydroxypropionic acid 11% 15% 18%  9% 15% 18% Acrylic acid 85% 80% 77%87% 79% 76% Glucose  1%  1%  1%

Working Up 1

The bottom product of dehydration 1, which was rich in acrylic acid, wasfed to a suspension crystallizer (MSMPR “mixed suspension mixed productremoval” type as described in Arkenbout G. F. “Melt CrystallizationTechnology”, Lancaster, Technomic Publishing Company Inc. 1995) with avolume of 1 l and was cooled down to an equilibrium temperature of 0° C.with a temperature ramp of 0.25 K/min. The crystal suspension obtained,with a solids content of approx. 50 wt. %, was separated into crystalsand contaminated mother liquor via a conventional laboratory suctionfilter. Thereafter, the crystals were washed with purified acrylic acid(99.5 wt. %) in a weight ratio (crystals/purification liquid) of 5:1 andfreed from adhering impurities. The purified crystals were brought toroom temperature and thereby melted. An acrylic acid having a purity of99.5 wt. % was obtained by this procedure. The compositions of thephases (feed, crystals and mother liquor) are given in Table 2.

TABLE 2 Feed Crystal phase Mother liquor Water  3%  0.1%   5.9%3-Hydroxypropionic acid 11% 0.37% 21.63% Acrylic acid 85% 99.5%  70.5%Glucose  1% 0.03%  1.97%

Dehydration 7

Dehydration 4 was repeated, with the difference that the dehydrationproduct was let down directly after the pressure reactor 91 and has thecomposition given in feed 1, Table 3.

Working Up 2

Stage A)

The water-rich acrylic acid mixture of feed 1 was introduced into a 1 lstirred crystallizer and cooled down to a temperature of −18° C., closeto the triple eutectic point, with a temperature ramp of 0.25 K/min. Thesuspension was subsequently filtered over a conventional laboratorysuction filter. The crystals obtained were warmed to room temperatureand melted. The compositions of the crystals 1 and the mother liquor 1are listed in Table 1. The crystallization was carried out analogouslyseveral times in order to obtain an appropriate amount of product forthe following crystallization steps.

Stage B)

The crystal phase 1 of stage A predominantly comprising water andacrylic acid was introduced in turn into a 1 l stirred crystallizer andcooled down to a temperature of −10° C. with a temperature ramp of 0.25K/min. Crystallization of this virtually binary system of water andacrylic acid was interrupted before the binary eutectic was reached, sothat essentially water crystallizes in a targeted manner. Thereafter,the suspension obtained was drained out of the crystallizer andseparated into crystals and mother liquor over a laboratory suctionfilter. The compositions of the mother liquor 2 and the crystal phase 2are likewise listed in Table 3. This crystallization was likewisecarried out several times in order to provide an appropriate amount foruse for the following crystallization step.

Stage C)

The mother liquor 2 of stage B with a virtually eutectic composition ofwater and acrylic acid was introduced into a stirred crystallizer andcooled down to approx. −15° C., below the binary eutectic point, with atemperature ramp of 0.25 K/min. The suspension of crystals of water andacrylic acid formed by this procedure was passed over a laboratorysuction filter and filtered. The compositions of the mother liquor 3 andthe crystal phase 3 are likewise listed in Table 3.

TABLE 3 Crystal Mother Stage A) Feed 1 phase 1 liquor 1 Water 80.0% 79.0% 80.4% 3-Hydroxypropionic  5.0%   1.0%  6.7% acid Acrylic acid15.0%  20.0% 12.9% Feed 2 = Crystal Mother Stage B) Crystal phase 1phase 2 liquor 2 Water 79.0%  99.5% 35.2% 3-Hydroxypropionic  1.0% 0.02%  3.1% acid Acrylic acid 20.0%  0.48% 61.7% Feed 3 = CrystalMother Stage C) Mother liquor 2 phase 3 liquor 3 Water 35.2%  38.0%30.1% 3-Hydroxypropionic  3.1%  0.00%  9.6% acid Acrylic acid 61.7%62.00% 66.4% An acrylic acid/water phase having an acrylic acid contentof 62% and a water content of 38% was thereby obtained.

The monomer solution obtained from the crystallization comprising 260 gof acrylic acid (62%), which was neutralized to the extent of 70 mol %with sodium hydroxide solution (202.054 g of 50% strength NaOH), 160 gof water (38%), 0.409 g of polyethylene glycol 300 diacrylate and 1.253g of monoallyl polyethylene glycol 450 monoacrylate are freed fromdissolved oxygen by flushing with nitrogen and cooled to the starttemperature of 4° C. When the start temperature was reached, theinitiator solution (0.3 g of sodium peroxydisulphate in 10 g of H₂O,0.07 g of 35% strength hydrogen peroxide solution in 10 g of H₂O and0.015 g of ascorbic acid in 2 g of H₂O) was added. When the endtemperature of approx. 100° C. was reached, the gel formed wascomminuted with a meat chopper and dried in a drying cabinet at 150° C.for 2 hours. The dried polymer was coarsely crushed, ground by means ofan SM 100 cutting mill with a 2 mm Conidur perforation and sieved aspowder A to a particle size of from 150 to 850 μm.

100 g of powder A are mixed with a solution comprising 1.0 g of ethylenecarbonate (EC), 0.6 g of Al₂(SO₄)₃×14 H₂O and 3 g of deionized water,the solution being applied to the polymer powder in a mixer by means ofa syringe with a 0.45 mm cannula. The powder A coated with the aqueoussolution was subsequently heated in a circulating air cabinet at 185° C.for 30 minutes. A powder B was obtained (particles sizes: on 150 μm meshwidth 13%, on 300 μm mesh width 15%, on 400 p.m mesh width 12%, on 500μm mesh width 15%, on 600 μm mesh width 20%, on 710 to 850 μm mesh width25%). The properties of this powder are given in Table 4.

TABLE 4 Coating with Properties EC Al sulphate CRC AAP_((0.7 psi))Powder [wt. %] [wt. %] [g/g] [g/g] A 0 0 33.8 20* B 1.0 0.6 29.6  23.7*determined at 0.3 psi

Test Methods

Melting Point Determination VIA DSC

Apparatus:

DSC 820 from Mettler Toledo with FRSS ceramic sensor and silver furnace(−150 to 700° C.)

Principle:

The temperature (the temperature range) of the phase transition from thesolid into the liquid state is determined. In practice, a sample of thesubstance to be analysed is heated under atmospheric pressure, thetemperatures of the start of melting and of complete melting beingdetermined. Differential scanning calorimetry (DSC) is employed as themethod. According to DIN 51 005, this is understood as meaning athermoanalytical method in which by measuring the temperature differenceon a defined heat conduction zone between the sample and reference,which are subjected to the same temperature programme simultaneously,quantitative recording of the difference in heat flow is made possible.This heat flow is measured and plotted as a function of the referencetemperature. The proportionality constant K, i.e. the calibrationfactor, depends on the heat resistance and therefore depends on thetemperature. It must be determined by experiment. Melting is associatedwith an endothermic change in enthalpy, crystallization (freezing,solidification) with an exothermic change.

Melting Point:

That temperature at which the transition between the solid and liquidphase takes place under atmospheric pressure is called the meltingtemperature. Under ideal conditions this temperature corresponds to thefreezing temperature. Since in many substances the phase transitiontakes place in a temperature range, this transition is also often calledthe melting range. For pure substance there is only one effectivemelting point, i.e. only a single temperature at which the solid andliquid are in equilibrium. All impurities and admixtures of othercomponents or e.g. a second or third main component result in a meltingrange in which the two phases, melt and solid, occur simultaneously withdifferent compositions over a certain temperature range. The start ofmelting is indicated here with the extrapolated onset temperature. Themelting range indicates the effective two-phase region in which the meltand solid are in thermodynamic equilibrium. It is often not sufficientto describe the melting of a substance only with the melting point.Recording of the entire course of melting is indicated above all if thesubstance is not pure but is a mixture of several similar substances,other substances or modifications (polymorphism), and possibly showssimultaneous decompositions.

Plastics with a wide crystallite melting range also requiredetermination of the entire melting behaviour.

Evaluation:

The DSC measurement data are shown on a curve in which the difference inheat flow or the temperature difference assigned to this is plottedagainst temperature or time. In this context, differences in heat flowfrom endothermic processes are plotted in the positive ordinatedirection, and those from exothermic processes in the negative ordinatedirection (see DIN 51007). For characterization of a melting peak,according to DIN 51004 the following temperatures can be used (see FIG.4): peak onset temperature, extrapolated peak onset temperature, peakmaximum temperature, extrapolated peak end temperature and peak endtemperature (for terminology see DIN 51005). The extrapolated peak onsettemperature is counted as the melting temperature. The peak maximumtemperature (with an exponential drop of the peak flank) corresponds tothe clear melting point, i.e. the end of the melting range where themolten substance is no longer clouded by suspended crystals. It is alsocalled the melting point of the last crystals or liquidus point.

Definition of characteristic temperatures of a peak (according to DIN51004) In FIG. 4:

-   Ti denotes the peak onset temperature; where the measurement curve    starts to deviate from the extrapolated initial base line-   Te denotes the extrapolated peak onset temperature; where the    auxiliary line through the ascending peak flank intersects the    extrapolated initial base line-   Tp denotes the peak maximum temperature; where the maximum of the    differences between the measurement curve and interpolated base line    lies (this is not necessarily the absolute maximum of the    measurement curve)-   Tc denotes the extrapolated peak end temperature; where the    auxiliary line through the descending peak flank intersects the    extrapolated end base line-   Tf denotes the peak end temperature; where the measurement curve    reaches the extrapolated end base line again

In practice, the base line is interpolated between the peak onsettemperature and peak end temperature by various methods (see DIN 51007).For the definition of the extrapolated peak onset and peak endtemperature, in most cases a base line interpolated linearly between thepeak onset and peak end can be used with sufficient accuracy. Theauxiliary lines are laid through the (almost) linear part of the twopeak flanks either as inflectional tangents or as regression lines. Thedifferentiation between the two methods is of no significance in(calibrating) practice, since the differences which result are muchsmaller than the repetition scatters of the measurements. The meltingpeak is conventionally characterized by the extrapolated peak onsettemperature.

Determination of the Phase Diagram of a Three-Component Mixture

After the melting points or the melting ranges, i.e. the two-phaseranges, have been measured for various mixtures of a three-componentsystem, these can be plotted in a three-dimensional triangular diagram.This can be visualized on the basis of appropriate evaluation software,e.g. Windows Excel®. The resulting areas of the melting points measuredgenerate so-called eutectic troughs in the eutectic system, whichintersect at a defined point, the so-called triple eutectic. FIG. 5 ashows a planar isothermal section through a three-dimensional triangulardiagram. FIG. 5 b shows a corresponding plan view of the phase diagramwith the triple eutectic. FIG. 6 shows a three-dimensionalrepresentation.

Determination of the Retention

The retention, called the CRC, is determined in accordance with ERT441.2-02, where “ERT” stands for “EDANA recommended test” and “EDANA”stands for European Disposables and Nonwovens Association”.

Determination of the Absorption Against Pressure

The absorption against a pressure of 0.7 psi, called the AAP, isdetermined in accordance with ERT 442.2-02.

Determination of the Particle Size

The particle sizes are determined in the present case in accordance withERT 420.2-02, the sieves stated here being used.

Determination of the Phase Composition

The phase compositions during the crystallization are determined by HPLCand are stated in %.

LIST OF REFERENCE SYMBOLS

-   1 Feed for biomass-   2 Synthesizing unit-   3 Outlet for a fermentation solution-   4 Killing unit-   5 Outlet for a fermentation solution containing killed    microorganisms-   6 Filtration unit-   7 Discharge for solids separated off-   8 Outlet for a fermentation solution freed from solids-   9 Dehydration stage    -   90 Membrane/filter    -   91 First reaction vessel    -   92 Discharge line for aqueous phase containing at least partly        reacted hydroxypropionic acid    -   93 Second reaction vessel constructed as a distillation device    -   94 Discharge for water    -   95 Bottom product containing water, unreacted hydroxypropionic        acid and acrylic acid    -   96 Feed line for carbon dioxide-   10 Outlet for an aqueous phase containing acrylic acid, water and    optionally unreacted hydroxypropionic acid-   11 Further device constituents, for example    -   111 Further filtration units    -   112 Protonation means    -   113 Adsorption means    -   114 Dewatering units-   12 Feed line for an aqueous phase containing acrylic acid, water and    optionally unreacted hydroxypropionic acid-   13 Purification unit    -   131 First crystallization device    -   132 Discharge line for the crystal suspension obtained in the        first crystallization device    -   133 First separating region    -   134 Melting unit for melting at least a part of the crystals        obtained in the first crystallization device    -   135 Conveying element, e.g. pump    -   136 Discharge line for the crystals separated off from the        mother liquor in the first separating region and essentially        containing water and acrylic acid    -   137 Melting unit    -   138 Discharge line for the crystal melt obtained in the melting        unit    -   139 Second crystallization device    -   1310 Discharge line for the crystal suspension obtained in the        second crystallization device    -   1311 Discharge for the crystals obtained in the second        separating region and essentially containing water    -   1312 Second separating region    -   1313 Discharge line for the mother liquor separated off from the        crystals in the second separating region    -   1314 Third crystallization device    -   1315 Discharge line for the crystal suspension obtained in the        third crystallization device    -   1316 Third separating region    -   1317 Melting unit for melting at least a part of the crystals        obtained in the third crystallization device    -   1318 Conveying element, e.g. pump    -   1319 Discharge line for the mother liquor separated off in the        first separating region-   Discharge line for purified acrylic acid

1. A process for the preparation of acrylic acid, comprising the processsteps: (a1) provision of a fluid F1 containing a hydroxypropionic acid,aqueous phase P1, (a2) dehydration of said hydroxypropionic acid to givea fluid F2 containing acrylic acid, aqueous phase P2, (a3) purificationof said fluid F2 containing acrylic acid, said aqueous phase P2, bycrystallization to give a purified phase.
 2. The process according toclaim 1, wherein said fluid F1 is obtained by a process comprising theprocess steps: i) preparation of 3-hydroxypropionic acid from abiological material to give an aqueous phase containing hydroxypropionicacid and microorganisms, ii) optionally killing of said microorganisms,iii) separating off of solids from said aqueous phase.
 3. The processaccording to claim 1, wherein said fluid F1 is aqueous, and has acomposition comprising (C1-1) from about 1 to about 40 wt. % ofhydroxypropionic acid, salts thereof, or mixtures thereof, (C1-2) fromabout 0.1 to about 5 wt. % of inorganic salts, (C1-3) from about 0.1 toabout 30 wt. % of organic compounds which differ from hydroxypropionicacid, (C1-4) from 0 to about 50 wt. % of solids, and (C1-5) from about20 to about 90 wt. % of water, wherein the sum of components (C1-1) to(C1-5) is 100 wt. %.
 4. The process according to claim 1, wherein saiddehydration comprises a reactive distillation.
 5. The process accordingto claim 1, wherein said dehydration is carried out in at least tworeactors, comprising the following process steps: i) heating of saidaqueous phase P1 in at least one first reactor R1 in the presence of ahomogeneous or heterogeneous catalyst to give a first aqueous fluid F1-1containing acrylic acid under a pressure Π1; ii) introduction of saidfluid F1-1 into a further reactor R2; and iii) heating of said fluidF1-1 introduced into said reactor R2 in the presence of a catalyst undera pressure Π2 to give a fluid F1-2, wherein said pressure Π2 and saidpressure Π1 are not equal.
 6. The process according to claim 5, whereinsaid pressure Π2 is lower than said pressure Π1.
 7. The processaccording to claim 1, wherein said dehydration is carried out in a CO₂atmosphere, or an inorganic acid, or both.
 8. The process according toclaim 1, wherein said crystallization is selected from the groupconsisting of a suspension crystallization or a layer crystallization.9. The process according to claim 1, wherein said aqueous phase P2 iscooled to a maximum to the temperature of the triple eutectic point of acomposition of acrylic acid, water and hydroxypropionic acid.
 10. Theprocess according to claim 7, wherein a first crystal phase containing:i) at least about 5 wt. % of acrylic acid, ii) at least about 40 wt. %of water, and iii) at most about 10 wt. % of hydroxypropionic acid isobtained.
 11. The process according to claim 8, wherein the purificationis carried out by an at least two-stage crystallization.
 12. The processaccording to claim 1, wherein process step (a3) comprises the followingprocess steps: (a3-1) crystallization of said fluid F2 in a firstcrystallization stage to give a crystal phase K1 and a mother liquor M1,wherein said crystal phase K1 comprises: i) from about 5 to about 60 wt.% of acrylic acid, ii) from 39.9 to about 95 wt. % of water, and iii)from about 0.1 to about 10 wt. % of by-products which differ from waterand acrylic acid, and wherein the sum of the amounts by weight ofacrylic acid, water and by-products is 100 wt. %, (a3-2) separating offof said crystal phase K1 from said mother liquor M1, (a3-3) melting ofsaid crystal phase K1 from said first crystallization stage, (a3-4)renewed crystallization of said melted crystal phase K1 in a secondcrystallization stage to give a crystal phase K2 and a mother liquor M2,wherein said crystal phase K2 comprises: i) from about 8 to about 35 wt.% of acrylic acid, ii) from about 60 to about 90 wt. % of water, andiii) from about 2 to about 5 wt. % of by-products which differ fromwater and acrylic acid, wherein the sum of the amounts by weight ofacrylic acid, water and by-products is 100 wt. %, (a3-5) separating offof said crystal phase K2 from said mother liquor M2, (a3-6)crystallization of said mother liquor M2 in a third crystallizationstage to give a crystal phase K3 and a mother liquor M3, wherein saidcrystal phase K3 comprises: i) from about 25 to about 55 wt. % ofacrylic acid, ii) from 44.5 to about 70 wt. % of water, and iii) fromabout 0.5 to about 5 wt. % of by-products which differ from water andacrylic acid, wherein the sum of the amounts by weight of acrylic acid,water and by-products is 100 wt. %, and (a3-7) separating off of saidcrystal phase K3 from said liquor M3, a purified phase containingacrylic acid crystals being obtained.
 13. The process according to claim12, wherein the mother liquor obtained in process step (a3-2) is fedback into step (a1).
 14. The process according to claim 1, wherein saidpurified phase has a purity of at least about 40% with respect to saidacrylic acid.
 15. The process according to claim 1, wherein saidhydroxypropionic acid in said fluid F1 or the acrylic acid in said fluidF2 is converted into its protonated form by the addition of acids beforecarrying out at least one of process steps (a2) or (a3).
 16. The processaccording to claim 1, wherein said hydroxypropionic acid is2-hydroxypropionic acid.
 17. (canceled)
 18. A device for the preparationof acrylic acid, comprising the following units and stages, wherein saidunits and stages are connected to one another by fluid-carrying lines:i) a synthesis unit for the preparation of hydroxypropionic acid, ii) adehydration stage for conversion of the hydroxypropionic acid intoacrylic acid, followed by iii) a crystallizing unit. 19-31. (canceled)